From what I've read on the game it supports CrossFire, but not SLI, but who knows.
BTW, at max settings and 1080p the game is very choppy for me as well, even with the set up I have. It doesn't seen like the typical "lag" to me, it's more of a constant "choppiness", which just makes me believe the game is highly unoptimized. I just got the game through steam and I'm honestly tempted to uninstall it.
My brother with a 3.1 Ghz AMD dual core and a single BFG 8600 GTS has to play the game in windowed mode because it's so unoptimized he can't even run it at 800x600 in full screen mode. He doesn't have the most powerful setup but at least you'd think he'd be able to run it full screen.... stupid game.
t support SLI at first, but after the first patch, and a driver update it did. It was only left without sli support for a few weeks.
The game also isn't unoptimized there was probably something wrong with his system seeing how it couldn't run full screen. If a game can't run full screen it's usually because of a conflict with software on the system.