Ok, I'm running Mandriva 2007 on this machine, dual booting with Vista. I've got the dual monitor situation squared away, however, if I enable Compiz, it only works on the primary monitor. The secondary monitor gets the wallpaper of the bootloader and the only thing that'll show up on it is the mouse pointers when I move the mouse to that screen. Without Compiz loaded I have both screens set to 1920 x 1200 but with Compiz loaded it thinks I have one display running at 3360 x 1050. When I flip the cube around to the other desktops I can see the other half of the desktop that's being hidden rather than displayed on the second monitor. I've seen the GoobTube videos of people using Compiz in a dual display setup, but nobody bothers to tell how they got it working. Anyone got any ideas of how to fix this?

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The dual head Compiz setups you're seeing are probably NVidia TwinView based, one card, two screens pretending to be one. What you're looking for requires Compiz 0.7 which apparently has gotten support for this.

Compiz 0.6 doesn't seem to dig Xinerama.

According to the Compiz site v0.4 is the latest stable release and v0.5.2 is the latest development release.

That's the old site. Since they merged with Beryl, http://www.compiz-fusion.org/ is the actual site. Ignore the Compiz vs Compiz Fusion yapping, the community apparently doesn't even know what it wants.

I know you are using Mandriva, but you could try this:
http://ubuntuforums.org/showthread.php?t=583214

That's about enabling Twinview, which only works on dual-monitoring on a single card.