I'm trying to get this work as well, I've notice the game doesn't support ATI crossfire dual cards as well out of the box - Probably due to driver level, as I'm using ATI CrossFire Xtension to trick the Divinity 2 into other support crossfire games with some success. Example: Using/renaming the Divinity2.exe to Oblivion.exe; doubles your FPS - hence crossfire starts to work, but the shadows are screwy. I'm currently trying a bunch of different choices to see what might allow AA and Crossfire to work. I'll post my results when I find something interesting if at all.
I'm running a 4870x2 myself/Windows 7 Ultimate
