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.
7990 + 7970
690 + 680
7990 + 690
7970 + 680
some games work in 7990 and not work in 690,some games work in 690 and not work in 7990.
so play all games by inserting two company gpus in one system.