Its the xinerama extension... with both monitors connected you can usually get X to configure itself with:
And then test that file with the command they give you and see if that works, it'll be:
X -xf86config /root/XF86Config.net
It'll start X raw, without any windows... to get out of it: ctrl+alt+backspace. You might need to hand edit your XF file with the horizSync and VertRefresh of your different monitors... Xinerama is still a hand hack job, what's the card offhand?