I have done multiple GMs for the same campaign and scenario. For instance, by running a single urban game with GMs in charge of different areas of the city, and players basically changing game tables when they go to this or that part of town. Twice. Once with Cyberpunk 2020, and another time with the entire WoD, with the tables/GMs being running different locations and associated groups of supernatural inhabitants (werewolf pack, camarilla, sabbat, mages). The CB 2020 game might have had 20-30 players, and the WoD game probably had around 50 players. Both games worked admirably well.
Another multiple GM thing I've done, and this is in the long run, is basically running the same campaign world with GMs running their own areas in it. We've done this with the World of Darkness, many GMs we knew were basically picking one "by Night" City for themselves and running it for different players, but we would catch up regularly if we weren't playing at this or that other GM's games and basically assumed that all the events of one City carried over in all the cities of the same World of Darkness. So it was really our WoD with its own recent history and who's who and all that. I was running Paris by Night. There was LA by Night, Venice by Night, Cardiff by Night, Amsterdam by Night, Athens, Rome, Marseilles, Florence ... and I'm pretty sure I'm missing a few. It worked very well over ten years of simultaneous gaming.