Comment 36 for bug 294899

Great! But this is supposed to work out-of-the-box without any modified xorg.conf. Can you please try without any xorg.conf? It might be a challenge to fish out the log if the monitor goes off, but there are some ways:
On a virtual console (alt-F2 for instance, not the one where you run startx), mount your hard drive on /mnt, then run:
 sleep 60; cp /var/log/Xorg.0.log /mnt/var/tmp/ ; sync
Now you have 60 seconds to switch to the other screen (alt-F1) and run startx. Wait until the time has run out before rebooting. (Try the cp command without the sleep first just to make sure it can copy to the hard drive, before you set it up and run startx.)

BTW, the conf file in your last comment turned out to be the log instead...