I recently tried running 2 monitors on my machine (running Xubuntu 12.04), and whilst the second monitor was detected, it would only mirror the main monitor.
Out of curiosity, I booted Ubuntu 12.04 off a USB stick, and within the settings UI, I could select not to mirror the monitors, and everything worked very slickly indeed.
I thought I'd be clever and grabbed the monitors.xml file from the config, but that doesn't appear to exist in Xubuntu. So much for that idea.
Anyone know where/how I can set the second monitor up properly. I just want it to run as an extension to my desktop on the right.