I've got two NVIDIA 8800GTS cards on a ATI P5Q Deluxe. Both cards appear correctly in the device manager, but I don't get any options to extend the display onto the 3rd and 4th port (the ones on the second card) in the NVIDIA control panel or windows. I'm running Win 7 x64, and I'm trying to run four monitors, no SLI. I've seen several posts that say this is possible, I just can't see any way of extending the display to the second card.
Just so I clear a few things up to make this easier, and possibly solve your issue, I'll ask you a few questions:
Do you have the SLI bridge attached, or have SLI enabled in the Nvidia control panel? You will want to disable SLI and probably remove the bridge as well for your use.
Also, did you right click the windows 7 desktop, click screen resolution, then click detect? This should show all the displays you currently have hooked up and then allow you to organize them.