One of the interesting advancements in DirectX 12 is the way it allows graphics cards from different manufacturers to be paired together. Traditionally if you wanted a multiple GPU setup, you had to get two cards from the same manufacturer and often they had to be the same model too.
But DX12 is giving developers much more control over accessing and manipulating GPUs, and one manifestation is the freedom to pair different GPUs. It will still take a few years before the real-world benefits become much more commonplace, but the first benchmarks are in and the results are highly intriguing.
Anandtech has run the figures using Ashes of the Singularity, the real-time strategy game which is becoming synonymous with any mention of DX12 these days (until Fable Legends comes out, anyway).
The tech hub has run a range of combinations on a special build of the game provided by developers Oxide. It's special because it's the first version that supports the multi-GPU feature -- one of DX12's two multi-GPU modes, anyway -- meaning that you wouldn't be able to run this through your own system yet.
In any case, the combinations range from the high end to the low end, from a R9 Fury X and a GTX 980 Ti at 4K all the way down to a GTX 680 and a Radeon HD 7970. It's the low-end benchmarks that I find particularly fascinating, although it's worth noting that the 2012 GPUs got absolutely smashed when they tried to run Ashes at 2560x1440 (with the GTX 680 being the lead card).
Here's a video of Ashes running when the R9 Fury is taking the lead with a 980 Ti in tow. Those lasers look pretty.
You can read the full list of results and a highly technical breakdown, as well as a short history of DX12 and the problems with alternate frame rendering, the technique currently used by AMD and NVIDIA to make Crossfire and SLI setups function.