If your mobo has the correct expansion slots you can run multiple video cards at once. Each video card can run a monitor or 2 for an extended desktop. Windows xp and windows 7 can mix and match most any vendors cards, windows vista will only work with multiple cards that can run the same driver. If your using onboard video very few cards can use both onboard and a video card at the same time.
The easiest way to run 2 different tvs from one computer is to try to find 1 video card with all the outputs you need.