Yesterday night i was going for the same thing, borrowed a monitor from my bro to see how a dually plays. I could not seem to get both monitors to display correctly in any games. Warhead would have one black monitor, TF2 would only display the primary , MW2 would do the same . I had it set to span across both monitors and it would do so in windows , but not in game. I am certain i had the correct settings in Nvidia control panel but upon doing some googling, there was a few options people mentioned which i did not see. I looked in Windows display properties as well as Nvidia's. Even so, i did not find much regarding this as i would expect its not too much of an issue for many.
My drivers are current with nvidias latest 196.xx on a GTX260.