The Dimachaeri, a type of Roman gladiator that wielded two knives or gladius. Given they, like all gladiators, were entertainers, this trope's Rule of Cool stats can be inferred to date back to at least ~200-300 AD.
For an era, fencers would often wield a main-gauche in their off-hand. In fact, the word main-gauche means "left hand." It was primarily used for parrying.
The most popular version of this would be from the Spanish Verdadera Destreza ("True Dexterity", or "True Skill"), in the style known as espada y daga ("sword and dagger"), which made Spanish and Portuguese swordsmen be largely feared and respected during the 16th and 17th centuries. However, contrary to popular belief, the dagger was used less for attacking than for parrying, as well as for trapping the opponent's blade in combination with the main sword, usually using lever motions (although it would be still used to slash and thrust if the chance appeared by itself, obviously). Dual-wielding eventually lost popularity among fencers in favor of a single lighter, faster sword.
Also, although Iberian fencing always used the free hand for something, the dagger was only one of several options, which also included a buckler called broquel or rodela (rarely seen in swashbuckling media, despite it was even more popular than the dagger in certain contexts). Oftentimes, the hand was used to hold the user's cloak or cape, which would be used both defensively as a makeshift shield and offensively by throwing it over the opponent's blade to trap it. Even the hat could be a weapon on the free hand by throwing it against the opponent's face as a strategic diversion.
Several Italian schools of the late-medieval/early-Renaissance teach dual-wielding sideswords. Florentinian schools were particularly famous for it, though the style shows up in Bologna and Milan as well.
In terms of using two actual swords at once, there is a master named Giacomo di Grassi who wrote on the subject, using rapiers. His conclusions were the following: teach only to the most experienced, who have to be practically ambidextrous. Don't use in war or against armored opponents. In general, defend with one and strike with the other. It's often argued that he only put this in as a curiosity for the rich.
German dagger style that uses a pair of cleaver-like weapons, the use of a variety of daggers (not just the main-gauche) in fencing from the decline of shields down to the smallsword. (it truly became useless with the foil, as the foil was too small and light to effectively parry with a dagger. Most modern knives are heavier than a foil would be anyway), and whole schools of combat around sword and buckler (or sword and cape) that utilize the normally defensive weapon as an offensive weapon in specific cases.
Pirates did this back in the day. The key was that the second sword was very light and designed specifically for quick and safe sheathing, allowing them to be put away instantly if they needed to use their other hand.
Flankirovka is a Russian Cossack dance style involving the impressive control of two swords (usually the Shashka swords from which the style gets its alternate name). Whether it would work in combat is a moot point; swordmasters look upon it, rather sniffily, as show-off windmilling. A mistress of the art is Russian performer Kseira Rogers, who performs under the name of Rang. Watch her here''.
Pal Kinizsi, a legendary captain of King Matthias Corvinus of Hungary's army was often described by chronicles as always riding into battle wielding two long swords, however the few portraits remaining of him always depicted him with arming swords, which would be a bit more believable.
Also including the "Shaolin Pens" (a pair of needle-like weapons, usually attached to rings worn on the middle finger of both hands), fans (also used singular), dao (also used singular), axes (also used singular), maces (also used singular), and on rare occasions (mainly demos by Northern Shaolin) dual spears, staves, or chains.
Twin hook swords and the chicken sickles. The hooked ends are used to hook and trap the opponent's weapon as well as counter shields and slashing weapons such as dao. The tail ends are used as daggers. The crescent knuckle guards are sharpened, meaning you could slash with them as well. And of course the rest of the weapons are used like normal swords or sickles. The hooks can be used to link the two weapons and then swung.
There are many Chinese straight sword forms where you have a long straight sword in your right hand and your left hand clenches with index and middle finger pointed outwards. Those who are true enthusiasts of traditional Chinese martial arts know the real meaning of this hand sign. The left hand contains a hidden dagger◊ that can be used to stab the opponent when his weapon and concentration are distracted by a feint attack from your long sword.
Japanese martial arts
The traditional Okinawan weapon Kama (single handed short sickles) have forms with one in each hand. Both weapons are used for both striking and trapping the opponents weapon (the expectation was a kama weilder would be facing an opponent with a more traditional weapon such as a katana or bo).
Kobujutsu includes dual sai, dual jitte, and sai/jitte styles. This makes quite a bit of sense since an important quality of these weapons is to be able to trap swords in their tines. Once you've trapped the sword, you stab or bludgeon the enemy with the other weapon.
Samurai would occasionally use the two blades of their daisho, the katana and the smaller wakizashi at the same time, even although theoretically none of the two is designed to be employed with a single hand (at least, not to its fullest potential). Sixteenth-century samurai Miyamoto Musashi developed and employed an entire fighting style around this tactic.
Kendo has a style, nito-waza, of fighting with a long and a short sword. It's not particularly popular, however, due to its difficulty.
A modern branch of Japanese swordsmanship called Shinkendo apparently has techniques for dual-wielding katana, reserved for advanced students only.
Other Asian styles
Filipino Eskrima (also known as arnis, kali, or escrima), which was unsurprisingly influenced by Spanish Verdadera Destreza, teach users how to fight with a variety of two-weapon combinations, including twin sticks, stick and dagger, whip and dagger, and twin knives. In all combinations, however, both weapons are used to attack. The style is also noted for its use of a "live hand", wherein a fighter applies dual-wielding techniques to the combination of a weapon and a bare hand.
Korean armies used dual-wielding quite a lot. Their sword Hwando were shorter than Japanese katanas, but it meant they were easy to wield with one hand. One popular technique was dual-wielding swords that varied slightly in length, first striking with the shorter sword, than finishing the attack with longer one. Even if the first strike missed, the enemy would not expect the other sword to be much longer and be caught in surprise. They even went as far as to employ dual-wielding cavalrymen.
Haedong Kumdo, the modern Korean sword art, has the "ssangsu kumbup," the "two-hand sword technique."
The Sikh martial art, Gatka, teaches the dual-wielding of staffs for the purpose of fending off large crowds.
Some Mamluk warriors were trained to fight with two swords.
Other
Many cultures: shields. Shields are not usually passive defensive items and can be used to deliver vicious blows. Additionally, a shield is an active defense; the user actively maneuvers it to trap the opponent's weapons. Small and medium shields are often used to batter foes, such as punching with a buckler or smashing with a hoplon. Shields too big for this are still used actively, not merely passive held in the arm. Larger shields could be used to shove, bash (like driving the Roman scutum into the feet of a foe), or trap a weapon. From the Zulu nguni to the Norman kite shield to the buffalo hide shields made by the Comanche, they were almost never just held without being actively used.
There are a few exceptions that zig-zagged this trope, such as the pavise. Sometimes it would be planted down in front of a crossbowman or worn on the back to provide defense as they reloaded. However, it was often actively used if the crossbowman was charged or if other soldiers happened to be in hand-to-hand combat holding one.
, a type of Roman gladiator that wielded two knives or gladius. Given they, like all gladiators, were entertainers, this trope's Rule of Cool stats can be inferred to date back to at least ~200-300 AD.
