I'm running an nvidia card on a machine running a fresh install of 12.04. I'm using the drivers available from the main repos. I have the program 'nvidia-settings' available.
I have an x display on each monitor. However, when I switch desktops on one monitor, the desktop on the second monitor also changes.
This is a departure from the behaviour that I had on 10.10, where I could control each monitors desktop display independently.
Is there any way to get this behaviour back on 12.04?
There are a couple of settings that you can configure using the nvidia-settings. for an explanation of twinview and separate X screens here. It sounds to me as if you might have chosen twinview when configuring the nvidia-settings. You would need to go back in to the nividia-settings manager:
It will require that you log out, then log back in order for the configuration change to take place.
Hope this helps.