Re: [BugReport] Xinerama Broken in 190.42
I think its an xorg 1.7 issue. I was able to switch to the nouveau driver and get the same problem.
I haven't tried messing with the geometry like you suggest. I'm actually pretty partial to my separate screen non-xinerma setup; I was merely trying to enable Xinerama and see if a different problem I was having went away or not.