Originally Posted by Mussels
to me, it sounds like its dividing parts of the screen up - say if it was 2 cards, they'd each think they were driving half that resolution.
each card would be doing 1600x600 on its own monitor, as far as it was concerned
Nope, that's what SLI and Crossfire are already capable of doing, in some cases.
Lucid distributes the work into polygons. One GPU handles the characters whereas the other handles the environment. This way they don't have to load the same texture data so the system actually doubles the available video memory with two cards.
That's why you can couple different cards. They're doing different things.