At its most basic, multiple GMs simply need to know their region (which could be as simple as their dungeon(s)), and what PCs are available and up-to-date. The players manage their stable of characters. So if a particular group of players wants to go to Tim's Dungeon, they can pick from any of their characters that aren't still stuck in Matt's Dungeon.
If there are multiple towns/regions, then it's slightly more complex, but still the same principle: the PCs for today's specific adventure need to be available in within travel distance of today's specific adventure.
That's 4 columns on a spreadsheet: Player-Character-Town-Status (alive/dead/other long-term condition) w/ Date
(Or maybe add a "last used" date or something?)
Keep in mind, this is operating under the idea that there's no actual adventure in the town. If the GMs are juggling the same towns with NPCs, the same wilderness and dungeons, then things are gonna get messy. Even then, though, you could have "side treks" that allow the players to all pick Tim's Dungeon for this session, but Tim's not free, so Matt takes over as GM and runs a side trek (wilderness "random" encounter). Next time Tim's free, though, and those players are, then they do Tim's Dungeon, possibly skipping the initial journey.