The easiest explanation for these crossovers is that they take place in an alternate reality in which the universes are already combined. For example, 1995’s Galactus vs Darkseid: The Hunger (by John Byrne and Rick Taylor) takes place in a universe where Galactus has devoured most planets, leaving only New Genesis and Apokolips.
But as crossovers between DC and Marvel grew more common in the ’90s and 2000s, the alternate universe explanation no longer worked. Fans wanted to see the real heroes interact, which meant that writers needed to address the universal distinctions.
In the best of these crossovers, 2003’s JLA/Avengers and Avengers/JLA (written by Kurt Busiek, illustrated by George Pérez, and colored by Tom Smith), the DC villain Krona encounters Marvel’s Grandmaster while traveling across universes. The Grandmaster offers Krona a wager, which drives the JLA and the Avengers to visit each other’s universes, and leads them to combine and mutate.
But for the much lesser event DC Versus Marvel and Marvel Versus DC from 1996 (by various creators), the two universes are represented by twin figures called “The Brothers,” who kinda look like Robotech creations and cause heroes from DC and Marvel to zap back and forth. And, of course, fight.
For the Versus miniseries, the companies let the fans choose the outcome of the matches. Comic book shops carried ballots that fans would fill out and send in, deciding the winner of Wolverine’s throwdown with Lobo or Wonder Woman’s bout with Storm. Because these were fan polls, story logic didn’t matter. Wolverine trounced Lobo (mostly behind a bar, so we couldn’t see) and Spider-Man beat Superboy, after freaking out about the revelation that Kon-El is a clone… a touchy subject for Spidey in the ’90s.
Amid the clashes of heroes of two worlds, the Versus miniseries also featured the coming of a new hero, a man called Access.