There are two approaches to multiseat on Linux:
(1) Using multiple video cards (preferred).
Unfortunately, multi-seat capability in Linux using this approach is at an all-time low (bad state) right now, after working really well a few years back. This is due to refactoring in the X.org server and gdm. Until multi-card initialization in X.org gets fixed, multiseat is pretty much dead in the water. There's still a lot of interest in this approach, and once multi-card init is fixed, I think we'll see the remaining pieces -- gdm, audio, consolekit -- get sorted out in pretty short order.
(Note that multiseat is possible using the nvidia proprietary driver and a display manager other than gdm, if you have the right card combination. My 4-seat system, which worked beautifully in the F8 or F9 timeframe, currently works with 3 seats under F12 and the nvidia drivers + xdm; I can get two PCI video cards + one out of two PCI-e cards working).
(2) Using one video card with multiple outputs (lower performance). This works by starting a master X server that handles both screens as one display, and then layering a Xephyr server on each screen to handle that seat. This is a low-performance approach that should still work, but I haven't tried it for a long time.