This 1996 comic began a multi-issue crossover mini-series based on the premise of parallel universes. Fans voted on the outcome of the various super-matches. Later, an assortment of one-shot "Amalgam Universe" characters emerged, who combined features of specific Marvel and DC heroes and villains.
Super-heroes who team-up traditionally fight a pair of villains who have also decided to work together. A mid-1990s pairing of Superman and the Silver Surfer featured Mr. Mxyzptlk and the Impossible Man, who eventually turn on each other and during their battle, mimic a cross-section of characters from their respective universes.
Although DC and Marvel did not officially crossover until 1975, DC unofficially teamed up with their rival in a 1972 Justice League of America.