xrandr 1.4 multi-gpu works!
Today I discovered that a fundamental (well, for me) milestone in the linux desktop world has been achieved: with xrandr 1.4 it is effectively possible to seamlessly use multiple monitors attached to DIFFERENT VIDEO CARDS, with different resolutions, each with its own desktop, with the ability to move windows between screens AND full 3d acceleration on all the screens. All this with the open drivers.
I wanted to share this as information on this topic is nearly not-existent (all the articles I could find were about rendering offloading).
I tested it with a clean kubuntu 13.04 image, with 2 GTX 285. KDE detected the two screens without any configuration, and put a desktop on each one. I was able to run Unigine heaven 2.5 at high settings, windowed (1024x768) and move the window between the screens while it was running (15-30 fps). Simply amazing.