DX12 and windows 10 are both mgpu capable. The problem lies with the game devs and online gaming. To add mgpu capability can double the size of games, and with the current trend of online DLC taking 6+ hours for anything decent, servers like Steam, Uplay, Origin etc are not exactly happy to have such large bandwidth sucking server hogs. So initial games are kept as small as possible, with further DLC's added later.
Best option, sell both and get a 2070. Or if you are still pushing 1080p, sell either.