You can INSTALL two sound cards, but you can only have one input and one output at any one time.* Even using the input from one device to output to another device is dicey, and depends on drivers playing nicely together. [I'm looking at you, Creative]
Really, the best option is just ot get a card with all the inputs/outputs you need.
*The only exceptions are programs that let you select which card is the input/output, but I've seen some crazy things happen with some setups like this. And most programs do not give the ability to use the non-default windows sound device anyways...