Arthur and Jack are the same person
Jack Fairy as a boy became Arthur the young man, became Jack Fairy the performer, who just became Arthur again after glitter died. They are well connected in the film and there's not much other reason to have the Jack character exist. It would also explain how Arthur got the Oscar Wilde brooch.- Except that Arthur sees Jack in the street during the opening sequence, when he's running with the other teenagers, and other people see Jack too, so he can't be a figment of Arthur's imagination. So I guess that doesn't work.
