DirectX 12 will allow users to use AMD and NVIDIA GPUs in tandem in the same system, according to Tom's Hardware (thanks PCGamesN). This is unconfirmed. as they say this comes by way of "a source with knowledge of the matter," a source so circumspect it would not even invoke the term "DirectX," saying this info is about an "unspoken API." Here's what their tight-lipped source tells them:

We were also told that DirectX 12 will support all of this across multiple GPU architectures, simultaneously. What this means is that Nvidia GeForce GPUs will be able to work in tandem with AMD Radeon GPUs to render the same game – the same frame, even.

This is especially interesting as it allows you to leverage the technology benefits of both of these hardware platforms if you wish to do so. If you like Nvidia's GeForce Experience software and 3D Vision, but you want to use AMD's TrueAudio and FreeSync, chances are you'll be able to do that when DirectX 12 comes around. What will likely happen is that one card will operate as the master card, while the other will be used for additional power.

What we're seeing here is that DirectX 12 is capable of aggregating graphics resources, be that compute or memory, in the most efficient way possible. Don't forget, however, that this isn't only beneficial for systems with multiple discrete desktop GPUs. Laptops with dual-graphics solutions, or systems running an APU and a GPU will be able to benefit too. DirectX 12's aggregation will allow GPUs to work together that today would be completely mismatched, possibly making technologies like SLI and CrossFire obsolete in the future.

There is a catch, however. Lots of the optimization work for the spreading of workloads is left to the developers – the game studios. The same went for older APIs, though, and DirectX 12 is intended to be much friendlier. For advanced uses it may be a bit tricky, but according to the source, implementing the SFR should be a relatively simple and painless process for most developers.