https://bugzilla.kernel.org/show_bug.cgi?id=37362
--- Comment #4 from Alex Deucher alexdeucher@gmail.com 2011-06-13 16:37:27 --- (In reply to comment #3)
How can I check if the GPU is MUXes or MUX-less ?
Double check with your OEM, but being that your radeon card has no displays listed in its configuration, I'm pretty sure it's MUX-less.
Should I do more tests or wait ?
There's not much else you can do until the X server gets support for split display and rendering.