It is not impossible, but it can be a challenge.
XP (which you are running) only supports one active graphics driver. If you add two different graphics cards which need different drivers then it will not work on XP.
Note that this was a sane choice made over a decade ago when XP was designed. Back then using multiple displays was much rarer. For the very few who needed it you could work around it by using a dedicated graphics card with multiple outputs (usually a Matrox card) or with multiple identical cards and a driver which understood multiple cards.
Newer windows versions (Vista, win7, ...) allow you to load multiple graphics drivers at the same time. This means you can run different cards. (E.g. one Nvidia and one AMD based card).
This still can be messed up with broken drivers. However it should (and it often does) work.
No, not with XP.