I have an awesome idea.

I've played in and GMed in campaigns and/or one-shots with multiple GMs, under a few different circumstances:

Twice I've been involved in one-shots with multiple GMs: once as a player, once as co-GM. In both cases, the main use of the two GMs during play was to allow splitting the group without problems. The first one, in which I played, was a more-or-less traditional adventure setup, and the party splitting was incidental. The second one, in which I co-GMed, we had about twelve players on hand, so we decided to run a competitive scenario, with the players being divided into two groups who were trying to reach the same goal. For the first part of the game, we handled them separately, with the two of us periodically checking with each other to see where the other group was, etc. When the two groups finally came together, we worked together to resolve the massive argument followed by combat that resulted.

In a campaign setting, I've co-GMed in two ways: first, with us working together to create the setting, then switching off as GM for the campaign. This allowed both of us to play, and resulted in a greater variety of storylines for us.

Second, I've co-GMed with a shared world, where I ran campaigns in one part of the world, and another person ran campaigns elsewhere in that world. There wasn't that much communication required, but we did share our new stuff with each other, and every once in a while a character from one area would wander into the other.

Overall, my co-GMing experiences have been positive. I'm also of the type who likes to encourage players to "join in" by creating world background, subplot ideas, etc... I like the greater variety of ideas that comes out of sharing. Even when I don't have a formal co-GM, I'll often talk over my ideas for a campaign with one of my gaming friends, and "bounce" ideas off him/her to see what he/she comes up with.

--Travis