GDC 2016 - Stardock has some exciting stuff on its hands with Ashes of the Singularity, a DirectX 12-powered game that has an awesome benchmark for mixing AMD and NVIDIA video cards - but it's limited to just one game, for now.
Stardock has teased that it's working on a software solution, something that's part of DirectX 12, that will let gamers use multi-GPU support on DX12 games. Outside of AotS, multi-GPU support through DX12 isn't really a thing, but it could very well be in the near future.
Brad Wardell, Stardock CEO, explained to VentureBeat: "One of the biggest problems with games is that a new video card comes out from AMD and NVIDIA, and they're like [expensive], and you have to make a call. I like my video card. I can play most games on it, and I don't want to spend $800 on some new video card. But imagine, instead, hey, they're having a sale [using my GTX 760 as an example]. Hey, they're having a sale on an AMD 290 for $75. Wouldn't it be cool to put this into your computer and double your performance. You keep this in there [the 760]. You put this in there [the 290], and your games are twice as fast without doing anything else".