Why Can't I run 2 X servers on the same machine?

[Derek Tattersall]

I would like to set up 2 seats on my PC, with 2 mice, 2 keyboards and 2 
monitors. It is running f9, and the nvidia driver version 180.51.

I spent a few hours messing around with it, and I got it almost working.

I have an nvidia graphics GeForce 8500 card, and it works just fine as a 
dual-head card. I can also run 2 X servers on different VTs when the 
card is configured with dual heads. The problem comes when I try to run 
2 X servers, one on each of the 2 monitors.

So I modified the xorg.conf file to have 2 different server layouts, one 
for each monitor. It works perfectly when I try to run each server 
layout independently. It fails when I try to run tehm both at once. I 
get this error in my Xorg.0.log file:

/var/log/Xorg.0.log:(EE) NVIDIA(0): EVO Push buffer channel allocation 
failed

/var/log/Xorg.0.log:(EE) NVIDIA(0): *** Aborting ***

/var/log/Xorg.0.log:(EE) NVIDIA(0): Failed to allocate EVO DMA push buffer

/var/log/Xorg.0.log:(EE) NVIDIA(0): *** Aborting ***


Can anyone tell me what this error means?

Thanks.

Derek Tattersall