Right now I have two cards in my computer. A Radeon HD3870 and a GeForce 8400GS. I can start two seperate X-servers by executing:
Xorg -layout seat0
Xorg -layout seat1
There's no problem there. My actual question is: since the Radeon HD3870 is a dualhead card (ie. Multiple outputs, 2x DVI), could I start seperate X-servers on both outputs (with OSS drivers)? I know I can achieve this with Xephyr, but I don't want to rely on that.

This way I can remove the Nvidia card and save some power consumed by it.

Edit: Woops, this should be in 'X.Org & Mesa', shouldn't it?