Yes, it's entirely possibly to do that on a motherboard with two PCIe x16 slots. It's mostly a driver and configuration challenge.
Unless your "other card" is an ATI 5xxx series that you already own, you're most likely going to be better off just using the GTX 260 for everything. If you have a decent nvidia card, involving a second card so you can dedicate one to PhysX/CUDA just tends to generate more heat without increasing performance, and sometimes actually slows things down. Only thing that would make sense is if you already had a powerful ATI card as your main, but absolutely needed to add the nvidia technology after the fact.