Originally Posted by artem
What Aaron just says is that NVIDIA doesn't currently officially supports multiseat (NVIDIA GPU driving different video outputs).
To clarify, multiseat on -one card-. I've run multiseat on my workstation before (2+cards) playing all sorts of WINE and Linux games against someone else running on the other seat. It works perfectly. The problem the OP is running into, is that you cannot reasonably divide a card up to do two things at once that are unaware of eachother. This requires very complex driver support, and there are few use cases for such a solution.
If you're looking for massive multiseat, look elsewhere. Matrox makes cards supporting MANY seats and you can get alot of users on one beefy workstation that way.