Reproduction steps are simple:
- use computer with 2 nVidia cards (without SLI)
- first card is primary and used for desktop Xserver :0
- second card is configured manualy in xorg.conf (xorg.conf example here
- try to start second Xserver (ex: "QT_XFT=1 xinit -- :1 -sharevts -layout Myth -br -dpms")
I am almost sure, that problem is not in secondary xserver config. If I try to stop first :0 server ("service gdm stop") then I am able start :1 without problem.
Tested on ubuntu 11.04 driver 270.41.06
EDIT: fixed xorg.conf url