The primary video card is set to a slot. Sometimes in the BIOS you can change it, but the choices may not be obvious. Instead of displaying the name of the card it just has PCIEX_1, PXIEX_2, etc or something similiar to that.
If the adjustment isn't in the BIOS you have to physically swap the 2 cards.
Also you can run both video cards in your system at the same time on 1 monitor, and still connect a 2nd monitor anytime you want. In many cases it boost the performance, and in rare cases you get 90% of both cards. There should be a small cable to connect them on the back. This is called a crossfire configuration, or SLI in the nvidia world.