If it ain't broken you haven't tried fixing it enough!
AFAIK. The nVidia driver only supports one X server per GPU; multiple X screens not X servers. So same desktop manager.
Don't know what unity looks like on 2 screens.
You can not place icons on 2nd screen (not since 8.04), can not drag items between screens. Can move the mouse.
Can force apps to launch on either screen:
will launch vlc on 2nd X screen