Triplezock
Some friends of mine did this with vampires, werewolves and mages several times on conventions...
A few important things:
- Use an adventure with a strict timeline. Plan e.g. for 10 hours (if you got that much time) and include some events and actions of NPCs at certain times. That will keep the groups from drifting too much from each other in time. (means one group finishes equiping and gathering information in 20 minutes, the other wants to buy more specific things and they talk for 90 minutes.... )
- Keep the adventure in a small area. A village with a dungeon is often used.
- Every hour 5 minutes talk between the DMs WITHOUT the players listening.
For interaction: bring the groups together. This is fun though it usually ends with one group winning a fight on the marketplace... (yeah well, ya know, players...)
An adventure I heard about:
Evil rite shall take place at midnight. Good group knows that an evil priest got some artefacts for ritual magic and search for the guy, evil group is sent by another evil guy to steal the equipment... small village, huge graveyard, most villagers possessed....