It does not work like that. The only possible way to enable not fully enabled GPU's is through a BIOS flash, and only if they were not fused off. I do not believe the 690 is one that is even capable of being fully enabled beyond what it is already (I don't even think it has unused cuda cores). When it is possible, it is usually big news, the 6950's were one such card that could be fully enabled into a 6970.