FunkBGR said:
I've heard of people using multiple DM's for games - exactly what purpose does this serve?
Usually, the very general purpose is to keep a complicated game running more quickly and smoothly. How this is achieved varies widely.
In most games, the DM is a sort of bottleneck. Action can only proceed as quickly as the DM can describe and process events. If you spread that task over more people, you can keep things flowing. Think of it as multitasking, or processing the game in parallel, rather than in series.
Exactly who takes which tasks really depends on what you are trying to do.
I have worked on very large games of Paranoia, for example, with 14 players and three GMs. The main story was handled by the "lead GM", but as soon as combat breaks out, or as soon as someone steps aside to deal with something away from the central party (in Paranoia, this means pretty much constantly), the other GMs step up and help resolve the action.
In other games, having multiple GMs means being able to have a split party (esentially running two games at once). You can also arrange so that one DM is the "actions" man (the guy who resolves game mechanics), and another be the "acting" man (the guy who handles social relations and NPCs). Or one can play "arbiter" the other "the enemy" (meaning one handles all the mechanics, while the other handles the decision making for the monsters).