CrossSLI confusion between GTX 970 and R9 390X

In future, it may become possible to use both of them in tandem - there have been DirectX 12 tech demos showing SLI/Crossfire-like shared rendering between mismatched graphics cards.

How long it will be before it becomes generally-available and whether it will work in all/most DX12 games or only those specifically written to take advantage of heterogeneous multi-GPU support is, as yet, unclear. (For that matter, it may remain purely in the realm of tech demos.)