GTX 460 is in first pci slot, the gtx275 is in the 2nd.
Windows sees the monitor as following:

So 2 and 3 are on the 275.
Windows shows the background image on all monitors.
When you're not getting that, something is wrong with your setup.
I am using Windows7 premium, 64bit.
Haven't used my system with vista in this way, but should work as far as i know.