Install the video card and the drivers, and hook up a monitor to both jacks. Right click your desktop>properties>settings>advanced and see if there isn't a tab corresponding to your video card. If so, under that tab there MAY be options for dual monitors... this is the only way that I would know how it might be done.
I figured out the problem, the onboard video s agp, and my video card is also agp. Since you can only have 1 agp video card, my onboard video is cancelled out. If I had a pci video card it would work.