I guess technically you could run Ashes of the Singularity better, if you're really into that game by some lucky chance. It actually uses DirectX 12's multi-GPU support. The problem you run into is that pretty much nothing else does; software and hardware manufacturers have already mostly abandoned regular SLI and Crossfire support and supporting this feature is an even larger headache.
While you can have both cards in your system at the same time there is no reason to do so as all you are asking for is driver conflicts and performance headaches.
While you could use them in your PC at the same time, the only benefit would be paying a little more each month to your utility company, which probably ought not be considered a benefit.
I guess technically you could run Ashes of the Singularity better, if you're really into that game by some lucky chance. It actually uses DirectX 12's multi-GPU support. The problem you run into is that pretty much nothing else does; software and hardware manufacturers have already mostly abandoned regular SLI and Crossfire support and supporting this feature is an even larger headache.