Gpu set up look like this. 1x gpu=x16; 2x gpu=x16+x16=(x32); 3x gpu=x16+x8+x8=(x32); 4x gpu=x16+x8+x4+x4=(x32). so 2xgpu=4xgpu.
My recamend is use 1x6950 And1x660ti in 2xgpu setup.
sell one 6950.
I suspect you don't know what "16x" actually means in this context.
Also you cant SLI/Crossfire AMD and Nvidia cards (except when you hack drivers, and thats only for PhysX).