Originally Posted by DanTheBanjoman
Its a lot simpler actually. GPU A can't access the VRAM on card B fast enough. It would simply cripple memory performance and thus destroy performance.
as it stands, they just release cards with more Vram than they need, so that theres still leftover room for crossfire and SLI to use.
short of a new version of directX or oGL (and corresponding hardware) to be designed from the ground up to scale better in multi GPU, it wont happen.