I have a "multi-monitor" system on my desktop. My main screen is connected via DVI (a Samsung Sync Master, 1680x1050) and the other screen is a LG Samsung (full hd 1920x1080) that is connected via HDMI. Both are in the same graphics card, a Asus Geforce GTX 550 TOP.
The problem is that I can't use them both as I use on Windows or on my netbook. My netbook (an Asus 1215P) automatic detects all kind of screens under the settings that come with Ubuntu 12.04 (I think in english is called Screens, not sure). On my desktop, those settings only show one screen and call it as a "laptop screen".
I went to x server settings and my TV appeared there. I activated it, and set "separate x screen" (or something like that). I saved the settings and then I rebooted.
My TV screen was all grey and my mouse was a big "X". I couldn't move any window between screens.
I went back to x server settings and activated Xinerama. When I rebooted, multi-monitor was working, I could move windows between screens, and the login screen ever adapted to where my mouse was. But with Xinerama, my Unity 3D became 2D, and Ubuntu keeps giving me "system error" dialogs.
How can I set my multi-monitor system correctly with Unity 3D and no errors?
PS: My Ubuntu is 12.04 64-Bits