Back in 2010, a company called Lucid was promoting the concept of mixed GPUs and their technology was even adopted my some mainboard manufacturers for a short time. Lucid’s Hydra technology however did not stick around as the idea of splitting API calls and combining them before displaying on the monitor proved too complicated to be a problem-free gaming reality.
Fast forward to 2015 with Microsoft’s DirectX12 giving developers an API with a greater depth of hardware control, even allowing for cross vendor multi-GPU usage. Ryan Smith over at Anandtech benchmarked DX12’s multi-adapter performance with Ashes of Singularity testing combinations of current high-end and even older GPUs (Geforce GTX 600 series and Radeon HD 7900 series). The results are surprising with mixed vendor cards showing significant performance advantage over matching NVIDIA or AMD pairs.
Needless to say, this is just preliminary testing and implementation is completely up to the developer so not all upcoming games might see the same results.