As follow what I mentioned...
Dual screen has three setups.
1: dual xserver independent
2: dual xserver but glued with Xinerama
3:twinview:one xserver but stretched
If you use a video adapter in the newer market such as Nvidia Geforce4 or ATi Radeon VE above, you can setup up the dual screen in a single adapter but the same framebuffer.
Mode 1,2 have the Xvideo,OpenGL overlay in the "screen 0" which is shown up first(may be indexed in XF86Config-4 by the "Screen"). Mode 3 may fail to enable XVideo or OpenGL overlay.
BTW...the open source driver "nv" can't do the dual screen even though the hardware supports. Use binary "nvidia" instead. How about the ATis? I don't have the ATi cards so I can't tell what's going on. I will be very very glad if someone send me a card to test...
BTW...there is a easy sample in /usr/share/doc/nvidia-glx/examples/XF86Config.sample.gz.