I have 2 nvidia cards with three monitors Left (0), Middle (1), and Right (2). I have to use xinerama to make them all one desktop because xrandr only joins 0 and 1, leaving 2 as a separate desktop. My understanding is that xrandr can ONLY join two desktops, which is why I am using xinerama.
After MUCH trouble, I have three displays, all one desktop. But, there is a problem. Screens 0 and 2 work fine. Screen 1 is double-wide. It scrolls left-right as I mouse across it. The left half of the display is a mirror of screen 0. The right half of the display is what should be on screen 1. I've tried to make an image that shows what it is like:
I don't understand what can cause one and only one screen to mirror another screen. Further, how does it mirror one screen on half the display, but not the other half?
For reference, my xorg.conf is reduced to the bare minimum. My layout section only has Screen0 0 0, Screen1 Right of Screen0, Screen2 Right of Screen1, and xinerama on. My device sections only have the PCI address of the cards. My screen sections only have the device setting. So, there isn't a setting that would cause this, but I figure I'm missing a setting that stops it - if I just knew what that setting was.