The Whole 'Multiverse' thing goes back to the third century BCE whne the philosopher Chrysippus theorised that the world constantly died and was reborn across the ages. But it wasn't until 1952 that Schrodinger suggested that the several 'histoire apparently showen in his equations actually happened concurrently, rather than successively.

DC introduced the concept of Earth-2 in Flash of Two Worlds. But it was British fantasist Michael Moorcock who fictionalised the physics-based Multiverse from 1963 onwards in his 'Eternal Champion' series. But it is notable that most of the stories took place in only in one world - the worlds of Corum, Hawkmoon and Elric are, for the most part, self-contained. Admittedly, the Lords of Law and Chaos have the same names and purposes in both Elric and Corums' worlds, but their appearances, characters and hierarchy differ. Only at key poinbts do the various Realms interact.

AU stories have popped up from time to time in SF, even SF as poor and derivative as Star Trek and StarGate SG1. Of course, it is a staple of Fanfiction, especially the crossover stories that I perpetrate, which must ipso facto take place in Alternate Universes! But stories centring on the idea of the Multiverse itself have only been a recent development. With any luck they'll get bored with it before too long!