Just as the implementation of these different distributed rendering techniques will be left up to the developers, won't the use of mixed GPU's, especially cross-vender, be up to the GPU manufacturer? I doubt AMD and Nvidia will allow such a configuration. Suddenly the less expensive AMD flagship combined with an inexpensive Nvidia Cuda / PhysX card would gain a lot more traction I would imagine.
In order to be DX12 compatible your device must be able to compute. DX12 will treat all computer engines and a resource pool and distribute work among them. It doesn't matter if they're AMD, Nvidia, Intel, or whatever, as long as the device can support DX12.