Why would you even do that. An Apu is made for cheap budget gaming, maybe with one graphics card like the 6670. If you're going to buy 2 6670s, it would just be stupid. Just get the 650 ti or the 7770. Although, it would be a complete waste of money for your apu build.
If you really wanna do something like that, that won't waste your money, just get one 6670, and save up a lil to get a new processor and a whole new build.