But seriously, it's a technique called SLI (Scan-Line Interleave, or Scalable Link Interface) that essentially has two video cards working in-sync to draw your screen. It was first done awhile ago by 3dfx, and now it's making a comeback with PCI Express. Each card draws half of the screen by rendering every other row of pixels. Theoretically, your graphics capabilities should double.
ATI's multio gfx solution (crossfire) is simlar to that of Nvidia's SLI, but it has different render modes to Nvidia's multi gfx card offering. Untill recently, Nvidia required you to have the exact same gfx cards from the same company partner (so if you had one card from eVGA & one from leadtek they would not be compatible just untill now where I belive the latest drivers change this as I may recall reading).
ATI use a master and slave card concept with there multi card offering & offer different rendering modes (I can't be screwed expalining them all, maybe ati's website will hold more answers for you if your'e curious). having two gfx cards also means more power consumption, so you must use a good quality power supply of around 400w & up for multi gfx card systems.