They work together to render the graphics on your screen. Its called SLI (scalable link interface)
"An NVIDIA SLI system includes a PCI Express motherboard that supports two physical connectors that are capable of having two NVIDIA-based PCI Express graphics cards plugged into them. Joined by the NVIDIA SLI connector, the two graphics cards power one monitor, delivering earth-shattering
PC performance." -NVIDIAS SITE
excuse @ hardocp post , but they are good pics...
http://www.hardocp.com/image.html?im...8xMl9sLmpwZw== http://www.nvidia.com/page/sli.html