An RPGnet thread prompts this thought - have you played or run games with multiple GMs, at the table together or taking turns?
I haven’t done more than the occasional guest spot as GM (or a player doing that with me) since I was eleven or twelve.
(The exception being in prep, where I’ve discussed sessions and brainstormed with other GMs and players other than those involved, including Cat being co-“showrunner” in TWH, away from the table.)
A guest GM generally runs an adventure which doesn’t change much of the setting, and is expected to discuss possible ramifications with the regular GM. “I killed off the King, that’s okay, right?”
For example, I recently guest GMed in a Star Trek style SF game where I normally played the captain (conveniently missing after the first scene of this adventure) and ran the players through a ghost ship, a classic one-shot with no major effects once everything was sorted out.
The version when I was a kid was my first RPG, ye olde Fighting Fantasy, where there was no real continuity from one adventure to the other so anyone could have a go. Which was and would still be a good way for players to try their hand at GMing for a session or two.
For something with more planning involved, the GMs would naturally have to discuss things in more detail. An article in Fantasia magazine suggested various ways to deal with this to keep an ongoing game fresh - individual GMs might take various arcs, or various areas of interest within an ongoing setting, so one GM might be in charge of what the Dark Lord is up to while another deals with the Bickering Fiefdoms and another decides what the Assassins’ Guild is really planning. If the Dark Lord GM wants to put one of his agents in the Bickering Fiefdoms she discusses that with the Bickering Fiefdoms GM. It obviously involves trust between the multiple GMs.
Another option would be to have two GMs with different areas of interest at the table at the same time, to deal with a large enough group to justify this. The classic split is setting and rules, but one could be the NPC GM instead, for example. It would obviously help when the players split the party, as well, as long as the GMs can communicate.