Double Video Cards?

I’ve been presented with something that I’ve yet to have had the pleasure of coming across before. I have a user with a Dell GX1 and using the on board video adapter for his monitor. He wants to setup a second display for his pc. Would it be possible for me to setup a video card and use both display adapters at the same time? I was always under the impression that one blocks out the other. Am I wrong? The operating system is NT-04. Is this possible and how would I go about doing it? Thanks alot!