not every graphic card is capable of doing that, 6800 chipset did came with this type of configuration, BUT, with the bridge integration between cards, and as for nVidia technology, it only distributes screen processing into 2 equal parts (could be different as well), unlike others manufacturers, though equal distribution of the screen, but not completed half.
as if you are thinking of something like that, then u have to get mobo with 2 graphic slots and of course in case of nVidia 6800 series, a good 650W+ PSU