Hey, been recently thinking about buying a second AMD 6950. However I'm somewhat worried, mainly because I've seen that in Sharpshooters ENB it says that it's "crossfire-compatible".. What does this mean? Aren't any other mods compatible with Crossfire then? I'm kind of lost, any clarifications would be welcome!

Only mods like ENB that step between the game and the graphics card to do stuff would be affected. Quest, armor, texture and other content mods will not be affected.