Though I’m not quite sure if I’d be using/choosing Ashes of Singularity for a test like this myself, it is an interesting read. Anandtech posted some benchmark testing a feature in DirectX 12 called asymmetric multi-GPU. Mixing GPUs, even from different brands.

Basically you can mix DX12 class GPUs whatever brand you choose. The only requirement is that these cards support feature-level Direct3D 12_0. Now please understand your graphics card need to be DX12 compatible and of course the software as well. Using a Radeon R9 Fury X, and a GeForce GTX 980 Ti some results now have been posted.
 

It seems it works albeit only in alternate frame rendering. The performance increases are present but nothing close in comparison to 2-way SLI or 2-way Crossfire. It seems that mixing different GPUs from the same brand works as well thus a GTX 980 and a GTX TITAN Black would work as well. You can have a peek at the article here, below some benchmarks courtesy of Anandtech. Later on with some proper DX12 games, we'll obviously examine the technology as well.

Whether or not this will ever be an interesting enough technology only time can tell, both SLI and Crossfire are very refined and highly efficient.

Nvidia and AMD Cross Multi GPU Tested In DirectX 12