ASUS laptop and Xubuntu (11.10) running off the live CD will give me my dual monitors, but when I install Xubuntu I can't set up dual monitors. When I boot from the live CD both monitors come on even before it is fully booted. But in installed mode "desktop" and "grandr" says only one monitor. The "NVIDIA X Settings" show two monitors, (CRT-0- (ViewSonic VX1937), and DFP-0- (Apple Color LCD)), but the CRT-0- has no information in its window and the "Reset Hardware Defaults" button is grayed out.
Looks like this is a known issue. This is what I found on the Ubuntu Forums. Hope it helps you.