According to Toms Hardware one of the unexpected features of DirectX 12 is the ability to use Nvidia GPUs alongside AMD GPUs in multi-card configurations.
This is because DirectX 12 operates at a lower level than previous versions of the API it is able to treat all available video resources as one unit. Card model and brand makes no difference to a machine running DX12.
This could mean that the days of PC gamers having to decide between AMD or Nvidia could be over and they can pick their referred hardware from both companies and enjoy the best of both worlds. They will also be able to mix old and new cards.
However there might be a few problems with all this. Rather than worrying about your hardware optimization software developers will have to be on the ball to make sure their products work.
More hardware options means more potential configurations that games need to run on, and that could cause headaches for smaller studios.