Two years ago there was the render-nodes branch originally when David Airlie was working on allowing two X.Org Servers to run off one GPU, basically as sane multi-seat handling. Back in 2010 the work was in a "demoable" state, but never merged. The kernel work introduced the ability to create "render" device notes with a list of output resources, a hard-coded render node ssetup, and DRM mapping fixes for multiple device nodes.
Ilija Hadzic announced to the X.Org developers yesterday that he's revived David's render-nodes branch. He's currently seeking comments on the revived work per this mailing list message. "The following set of patches will revive the drm-render-nodes  branch that has been dormant in Dave Airlie's repository for some time. I rebased this branch to the latest drm-core-next and did some (hopefully useful) follow-up work. I fixed a few bugs, did a substantial cleanup, separated the experimental hard-coded stuff from general stuff and implemented all kinds of checks and protections from any ugly stuff that user space can send. I also have libdrm patches as well as a small test-utility program that can be used to create and remove render nodes from user space. I will send these in the next patch series."
Additionally from Hadzic, "At this time, I'd like to solicit comments and feedback and I'll be glad to rework the patches based on the feedback I receive. Note that although the patches are meant to work on any GPU, I have only tested this with Radeon hardware. If someone runs this with other hardware, I would be very interested in hearing about the result. Also, I have only tested this for multiseat-X use case. If someone tries this for GPGPU use case, I'd appreciate the feedback."
Let's hope this work will continue and end up being merged. Ilija Hadzic is the developer that previously sent over the DRM VBlank patch that led Linus Torvalds to go on a rant about why DRM has been problematic.