I previously posted an example of a map for a four-DM campaign; but really one does not need such a map for a multiple referee situation.
You could easily enough have one "city" with multiple campaign megadungeons within walking distance (or within or below the city itself).
Now being an obsessive freak I fret about the continuity, territorial and conflict of interest concerns with multiple DMs running parties in the same city ("God damn it Jack!! I invented the Snails Pace Inn and it definitely does not serve mead or have sexy serving wenches; weren't you paying attention when I DMed last week you fucking hack!"), but solutions come to mind:
- the city is just a generic, faceless entity devoid of NPCs, adventure or features; all of the action takes place in the dungeons with players shopping from a list while "in town."
- Only the features invented by a particular DM are available when that DM runs a session ("I understand that you wish to buy spells off of your mentor, but he's on vacation...").
- the city is divided into sections a la post-WW II Berlin with the action taking place in the quarter that "belongs" to the DM running the session ("Four kings?!? What kind of city is this place?!?").
Now any of the above conceits doesn't preclude any of the DMs from introducing their wilderness sandbox to the north-west/ north-east/ south-east/ south-west of "Central City," but they do remove it from being an immediate concern of any of the DMs and allows folks to haul their butts to the dungeon.