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. Solution: put that 2 gpus in x16 u get x32.
Z68/77 is going to force it in x8/x8 regardless. Just the limitation it has.
The 460 for PhysX I would imagine is capable. The most advanced game with it I can recall is Borderlands 2, which should be fine for a 460 Physx only. Personally I'd probably test it and see, not like it's hard since the you can just toggle where the PhysX is worked out.