Some gamers will do this in search of 1 more frame per second.
I'd rather see you pick off the list at this link.
-> For most I advise the single card. Most shops and owners can't setup more than one card.
It's between these graphic cards:
2 x 3 GB GDDR5 AMD® Radeon™ HD 7950 CrossFireX ,
Single - 4GB NVIDIA GeForce GTX 690 Grafikkort
Which graphic card should i choose?