I don't think it would be simple for developers on DX12 to implement multi-GPU on such different architectures, such as Intel's iGPU being compatible with GCN or Maxwell/Pascal.. would not end well.
This is where I see the advantage AMD have, they can have the same or similar uarch in their APU & dGPU, with DX12, the iGPU would no longer be useless sitting idle like our current Intel i5/i7 dGPU rigs.
@raghu78 I know some people here predict a looming death for dGPU but it won't happen. All that will happen is the enthusiasts market have to pay much more $ for their dGPUs to make it worthwhile. It also won't ever die off if AMD has their way with APU + dGPU (or even multiple dGPU) and it will all add up to greater performance.