You can't use your onboard video card and an add-in card at the same time, it's one of the other. Well you can, but only on motherboards that support hybrid CrossfireX. In most cases your onboard video is disabled when an add-in card is detected by the BIOS. To use dual monitors you will have to have a video card that supports it and connect both monitors to that video card. Then press the Windows key and the P key to extend your desktop onto the second monitor.
EDIT: Filling in your system specs would help.