I don't even know if red engine 4 is DX12 at all, might be DX11. If it is DX12 im doubtful it will have explicit multi GPU support over the PCI bus, im sure it will have SLI support witcher 3 does. But in terms of explicit muti gpu I doubt it, it does not seem to be a priority for any game developer. The only "game" I know that properly supports it is ashes of the singularity which is more of a benchmark than a game.