I have Xubuntu 12.10 and when i try to set 2 monitors if I expand the screen instead of mirroring it, the 2 screens become darker. I don't really know how to explain so I took a print-screen.
Thank you for your interest in this question.
Because it has attracted low-quality answers, posting an answer now requires 10 reputation on this site.
Would you like to answer one of these unanswered questions instead?