Working with others
In the LARP games most adventures I've played were small self contained affairs. Larger adventures or plots spanned over several days had to have anything OK'd by anyone whose "area" was involved - this works for large event systems like The Gathering and Isles of Darkness (LARP systems with thousands of players) the latter of which I was a GM in for far too many years.
For these games each faction/city/clan/etc in the system has several GM's and they are free to invent and create plot for their area as long as it doesn't impact the wider world (no gigantic floods) or another faction - as soon as they start doing that they need the approval of other GMs whose areas are involved. These systems DO have a concept however of GM hierachy and a master storyteller and these people can invent plot for all areas if they want.
For example in the IOD world system local areas have assistant GMs who can run stories if OK'd by the local GMs, if they want something that runs regionally then they need the OK of the regional GM and so on up to the master storyteller.
Personally I'd advise against this hierarchy system unless one GM is running games a lot more than others and the other GMs are just running now and then.
Note this becomes horribly bureaucratic in the larger systems, but for small tabletop systems this can be done much more easily and doesn't really need the hierachy system; the GM's simply have to adhere to these rules:
- Each GM is in charge of a nominal area or group of players and has places that are under their control.
- Affecting larger areas and groups needs approval from the other GMS - note this can always be done well in advance.
- Never step on anyone's toes without asking.
- Everything that has been previously stated is canon.
- You cannot interact even indirectly with one of your own other characters - they don't even exist as far as they are concerned *
- Wheatons Law
So if one GM runs an event where Baron Von Nasty has destroyed the city of Splat, the city of Splat remains destroyed, and all the consequences thereof. More importantly the information about why, how and what the Baron plans and what that they've discovered also remains canon.
In practice this has means that each time a GM swaps over they take up the "plot" with their group the world evolves all over and different stories and more culture emerges.
It is very helpful to have a common stomping ground or city where players can go, this needs to be a place where all the GMs can have their own NPCs and common NPCs that can be controlled (and maintained consistently) certain leeways should be drawn up as a loose agreement about what can be done to said city and any development of certain NPCs for that city or large destruction should be agreed.
E.g. In our major city of "Homesville" the great leader Bob is controlled by GM 1, but his seneschal is controlled by GM 2. Either GM can interact and use these NPCs but changing their personas, or major life events needs the other GMs approval (unless they've said they don't care)
In the end it's a bit of a "division of the spoils" of the world at hand and a "gentlemans agreement" (so to speak) not to muck around with other peoples stuff.
This is a fairly dynamic style of GMing and as such long-scale elaborate plots tend to emerge from NPC's goals (something I prefer anyway) rather than set-in-stone events.
* This is principally a LARP rule to stop players power-levelling between other characters, it can be an important consideration.