I have both monitors in portrait mode (i.e. they're vertical, not horizontal). Only one monitor shows an interface, the other is blank (white). I also do not know how to rotate the displays - I haven't seen an option for it.
OS: Linux-X86_64 (Ubuntu 11.10)
GPU: 2x Nvidia GeForce 550 Ti
Nvidia drivers: v280.13
Here is a screenshot of the one desktop that I can see (it's rotated by 90 degrees).