FUD Zilla reports the Radeon HD 3870 X2 graphics card and CrossFire X use Alternate Frame Rendering (AFR):
Alternate frame rendering is quite simple model, where the first chip renders the first frame, the second chip renders the second one and so on.
If you have as many as four cards, each card will render two images, something like card one renders the first and forth while card two renders the second and fifth frame, the third card renders the third and sixth frame and so on.
This should give AMD better performance and leaves space for eight way crossfire. In 3Dmark06 debugging mode we had a chance to see that four cards are rendering eight images virtually at the same time.