There's an overlap between the fandoms of Psychopath Diary and Strangers From Hell. Both of them are South Korean series about serial killers that aired in 2019-2020, though Psychopath Diary is much more comedic than Strangers From Hell.
Strangers From Hell: The fandom overlaps with the fandom of The Merciless, Beyond Evil, and The Devil Judge. Some Tumblr users have nicknamed the shared fandom for all four "Merciless Evil Devil From Hell".
Kingdom (2019) and Sweet Home (2020). Makes sense; they're both Korean horror series about an apocalypse turning people into monsters, and Kim Sang-ho plays a major character in both. Sweet Home (2020)'s fandom also overlaps with the fandom of Alice in Borderland, another horror series that aired on Netflix in December 2020.
Chernobyl and Game of Thrones share a fandom, largely due to the former's Win Back the Crowd effect on HBO subscribers after Game of Thrones wrapped up on a base breaking note. Despite the two show's completely different tones and subject materials, the internet has since exploded with crossover memes, from Valery Legasov being a real-life Azor Ahai, to a re-dubbing of the nuclear power plant explosion to the "Light of the Seven" score, to jokes that Chernobyl resolved all of the hanging Game of Thrones prophecies better than Game of Thrones itself did.
Game of Thrones and The Witcher also share a significant portion of their fan base, in part due to both being shows based on critically acclaimed fantasy books, and the series themselves sharing a fair portion of their production crew.
Joss Whedon's shows sometimes seem like they just have one big fandom. A good example of the "one big fandom" phenomenon is the Whedonverse Track at DragonCon, which is literally the fan track for ALL of Joss Whedon's stuff.
Joss Whedon's fandoms overlap with fans of Tim Minear, as he wrote and produced for several of Joss' shows, as well as creating shows with similar concepts which met the same fate as some of Joss'.
Veronica Mars fans overlap with Joss Whedon fans as well, likely because Joss himself was a fan of Veronica Mars (even making a cameo once) and because it features another snarky petite blonde title character who's a strong woman, similar to his Buffy the Vampire Slayer.
Tin Man and Alice - mostly because of their similar production staff and tone. Likewise with their literary ancestors. Baum was a fan of Alice in Wonderland.
Old-school Doctor Who and Blake's 7 were made by the same people and shared several actors, sets, and locations, so B7 became the original adult sister show to Doctor Who. Most B7 fans are also avid Doctor Who fans.
Blake's 7 has some overlap and friendliness with fans of Farscape and Firefly, as both can be seen as being "the 21st century Blake's 7", without being so similar as to arouse accusations of copying.
Sherlock and Doctor Who have a bit of overlap and crossovering going on (Sherlock overlapping more with Doctor Who than the other way round, due to a smaller fanbase) thanks to The Grand Moff being executive producer at both. Ever since Neil Gaiman wrote the very popular Doctor Who episode "The Doctor's Wife" for series 6, one which confirmed old fan theories, and contained a general display of awesome, and returned with "Nightmare in Silver" for series 7, the two fandoms have definitely gained a friendship.
The Smallville and Supernatural fandoms have a large amount of overlap, to the point where it could quite reasonably be argued that they share almost the same core base of fans. The fact that during the second half of the 2000s they almost always aired next to each other on the same network certainly helped this, as did the fact that they were the two powerhouse Sci-Fi series that were basically keeping the ungrateful CW Network afloat. The then-current network president's perceived treatment of the two shows as Network Red-Headed Step Children (in favor of promoting shows like Gossip Girl or the 90210Sequel Series as being the network's flagship programs) led to further unity between the two fandoms, which rightly or wrongly perceived themselves as being united against a common foe (namely, the perceived indifference on the part of the execs). The massive prevalence of fanfic crossovers between the two shows is also a testament to the unity between the Smallville and Supernatural fanbases.
Power Rangers and Super Sentai are two fandoms you'd expect to get into some variation on a Subbing Versus Dubbing brawl. And yet, that does not happen. Both the fans and production crews get along extremely well (to the point that they share sets and props, and for a couple of Super Sentai opening and ending sequences in the past few years, they filmed in New Zealand using the crew for Power Rangers), and forums for one will usually allow discussion of the other... The joint fandom can be expanded to include other Toku to various degrees; including Kamen Rider, Metal Heroes, and their variousAmericanadaptations (except maybe one we'd like to forget). The Japanese production house, Toei, has been encouraging this in recent years by including crossovers as part of their Milestone Celebrations.
Interestingly, Food Network shows generally have a lot of overlap in their fanbases, but not completely. While they are largely friendly towards one another, one celebrity chef's fans will often complain that several others on the same network are "too grating" or annoying.
SuperWhoLock seems to have adopted Merlin into the fold and the entire friendship is most noticeable when something bad happens in one of the fandoms (whether it be the demise of a character or the end of the series) and the other fandoms take to comforting its members.
iCarly and Austin & Ally, despite being on rival networks (iCarly on Nickelodeon and Austin & Ally on Disney) have a friendship based around the supporters of the two major canon relationships on the show, Carly/Freddie and Austin/Ally, via both being examples of the Just Friends.
The same can be said with both iCarly and A.N.T. Farm, in a case similar to the above example, but replace Austin/Ally with Chyna/Fletcher.
Star Trek fans are known for their running feuds with fans of other Sci-fi series/franchises; notable rivals being Star Wars, Babylon 5, and Doctor Who. But it seems there's a lot of warm regards, if not necessarily love, for fans of the televised Stargate shows especially Stargate SG-1. As different as the shows are, their similar takes on military science fiction/Space Opera and comparable places on the Sliding Scale of Idealism vs. Cynicism naturally means they'd appeal to a similar group. Also, it's clear the producers of Gate pay homage if ribbingly to Trek. To top it off, throw in the number of Trek actors who've wound up on one or the other Gate series and it's almost impossible to see these fandoms as rivals. The affection later spread to The Orville, with not a small number of Trek fans thinking it makes a better Star Trek series than Star Trek: Discovery, and a large number of fans watching both. For a long time on the Sci-Fi Channel, Star Trek: The Next Generation and SG1 were both staple shows, sometimes airing within hours of each other.
Star Trek and 24 fandoms got pretty cozy what with all the former Trek actors appearing in the show in big and large parts. The connection was especially strong with Star Trek: Enterprise which saw Gregory Itzin and Peter Weller doing both shows at the same time. (Itzin as President Logan on 24 while playing an alternate universe Starfleet Admiral on Enterprise. Peter Weller portrayed a Well-Intentioned Extremist on both shows!) The biggest connection was when Show Runner Manny Coto left Enterprise after it was cancelled and became a producer on 24. A Coto episode of 24 even saw a Bob Justman and a Rick Berman suggested to fill two open positions (Robert Justman and Rick Berman being longtime high-ranking members of the Trek staff.)
During The '70s, fans of Space: 1999 tended to also be fans of Star Trek reruns. Trekkies of that era sometimes credit the instant success of Space 1999 as one of the catalysts that convinced syndicated stations to also consider airing Trek reruns. In some areas, they were aired back to back.
There's quite a bit of overlap between fans of Downton Abbey and Call the Midwife. More specifically, fans who ship Mr. Carson/Mrs. Hughes on the former are very likely to also ship Dr. Turner/Sister Bernadette on the latter. It's been speculated that this is due to the tremendous amount of physical and emotional restraint on the part of each pairing.
Fans of Deadwood tend to also be fans of Justified and visa versa. Both are unconventional Westerns and many regard the later show as a kind of modern-day sequel to the former. It helps that they both star Timothy Olyphant and a bunch of Deadwood alums showed up on Justified to the point where fans talked about which Deadwood actors they'd like to see next.
After a very heated Fandom Rivalry with the Sherlock fanbase, the Elementary fanbase has found an Odd Friendship in the Hannibal fandom. Part of it may have something to do with Hannibal fandom avoiding the big three SuperWhoLock like the plague. Although Supernatural and Hannibal alone get along well, due to sharing a few actors and juicy crossover potential.
The close relationship between Elementary fandom and Sleepy Hollow fandom, which enjoys quite a bit of overlap in fanbases due to both shows having racially diverse casts anchored by well-rounded women of color protagonists. Sleepy Hollow also has fan overlap with Scandal for similar reasons, despite being in totally different genres.
Rome and Versailles have friendly comparisons. Likely due to being made by mainly the same people.
The West Wing and The X-Files fandoms often overlap, despite the shows' very different worldviews on the US government. The overlapping casts might have something to do with it.
Project Runway and RuPaul's Drag Race have a significant viewer overlap, and longtime viewers of Project Runway will recognize Santino Rice on the judging panel for Drag Race. Both shows also have a significant LGBT Fanbase due to the sheer number of queer contestants on both shows.
The "Shondaverse" of shows including Grey's Anatomy, Private Practice, Scandal and How to Get Away with Murder overlap rather easily as creations of Shonda Rhimes, as the casts of the shows usually have a good presence on social media and encourage viewership of each other's shows; same with the fans. The actors move between the shows very easily, acting as an extended family and also pushing their cast mate's outside projects with ease. Several of them are also married in real life to other cast and crew members of Shonda shows.
Agents of S.H.I.E.L.D. had a pretty big Fandom Rivalry with Arrow and The Flash (2014) back in the day as an extension of the Marvel/DC rivalry, but it strangely gets along well with the two's spinoff show, Legends of Tomorrow. It helps that both shows actually have a lot of similarities, being both ensemble shows about a varied cast who are overlooked by their larger Superhero setting, that started off on shaky ground but are generally seen as examples of Growing the Beard. They also both feature a well-regarded female lead, Daisy Johnson and Sara Lance, respectfully, who's evolved into being the lead character after the show initially presented a White Male Lead in the form of Phil Coulson and Rip Hunter as their leads (though in both cases, Phil and Rip are well liked and their relationship with Daisy and Sara is well-liked). Yet, despite being very similar in format, the shows avoid feeling like they're competing since they have vastly different tones (Agents started off with a similar tone to Legends, but became Darker and Edgier as it went on, while Legends got Denser and Wackier as it evolved).
Mister Rogers' Neighborhood and The Joy of Painting due to how both shows feature All Loving Heroes who are very calming, easy-going, and most importantly, selfless and highly respected. While the two shows have completely different premises (and more notably target audiences) from each other, both provide a very similar tranquil and healing feel.
There is a lot of overlap between the fandoms of This Country and People Just Do Nothing. While both shows feature very different settings- a rural West Country village and an urban pirate radio station respectively- both are spoof fly-on-the-wall documentaries following people who inhabit a tiny universe and are oblivious to how small and insignificant it really is. The creators of both shows also get on well and acknowledge the fandom overlap- one trailer for People Just Do Nothing has the Kurrupt FM boys giving a shout-out to "that other show that's like ours, the one in the country, but that one's better".
There's a far amount of crossover between the fandoms for Supernatural and Dark Angel owing to Jensen Ackles (Dean Winchester) having been on the main cast of Dark Angel's second season as Alec McDowell, and Dean and Alec having comparable Lovable Rogue / Handsome Lech personas. It's not uncommon to find crossover fanfics where either Dean and Alec are somehow the same person (despite Supernatural being set at the time it was produced and Dark Angel being 20 Minutes into the Future) or Alec and his psychotic Serial Killer twin Ben, who was also played by Jensen, were cloned from Dean (since the Dark Angel characters are genetically engineered clone brothers in the first place, which makes this an Obvious Crossover Method). Occasionally, you get a non-paranormal AU where Ben and Alec are simply Dean and Sam's younger brothers.
Donkey Hodie: Many older fans of the show seem to like Elinor Wonders Why, which also airs on PBS Kids and involves animals. There's also a few adult fans of Donkey Hodie who happen to like Pretty Cure. On the other hand, many children in the show's target demographic also like Cocomelon, another preschool show that also features songs in every episode.
Interview with the Vampire (2022) immediately picked up a lot of people from the Hannibal fandom, due to the overlap in horror, Gothic aesthetics, and toxic gay romance.
Hawk and Lestat: older, handsome, has a smooth torso, Hollywood Atheist, unashamed about being attracted to men, promiscuous, domineering, Seme (although they both have one scene where they're extremely aroused by their beloved dominating them), Consummate Liar. They both feel Love at First Sight and seduce respectively the man who becomes their Second Love...
The Journey of Flower: A brief look at the YouTube comments reveals that the series shares well over half of its fandom with Princess Agents, another series starring Zhao Li Ying.
Ruyi's Royal Love in the Palace and Story Of Yanxi Palace share a large portion of their fandoms. Unsurprising since both series are set during the Qianlong Emperor's reign and feature many of the same characters. note In fact, although the series are unrelated, Yanxi is almost a Perspective Flip of Ruyi: a character based on Empress Nara is a villain (named Hoifa-Nara Shushen) in Yanxi and the protagonist (named Ulanara Ruyi) in Ruyi, while a character based on Noble Consort Ling is the protagonist (named Wei Yingluo) in Yanxi and a villain (named Wei Yanwan) in Ruyi.
Goodbye My Princess and Novoland: Eagle Flag have an overlapping fandom, since the series are the same genre and have a similar tone (and also share an actor: Shawn Wei plays Gu Jian in GMP and Baili Ning Qing in NEF).
There is some overlap between the fandoms of No Ordinary Family and The Cape, as both are Short-RunnerReconstruction superhero shows that debuted around the same time and star some notable geekdom cult actors. They also balance each other out some in that No Ordinary Family has characters with real powers but no costumes or codenames, while the characters in The Cape have codenames but no powers (save perhaps Dice and Scales) and rely on technology or hard-learned fighting skills.
The Last of Us fans gets along with those of other live-action TV series.
Also with Fallout (2024), another post-apocalyptic show based on a popular video game. Fans of both series are happy for the positive reception of the shows as it continues the streak of good video game adaptations. It also helps that the show's respective co-creators, Craig Mazin and Jonathan Nolan, have high praise for each other's work with Nolan citing TLOU as paving the way for the Fallout.
