https://bugs.freedesktop.org/show_bug.cgi?id=64850
--- Comment #16 from Jakob Nixdorf flocke@shadowice.org --- I checkt again when the second monitor is switched off.
I booted with radeon.modeset=0, which caused the radeon module to not be loaded since I don't have UMS support in my kernel. With this both monitors work and I can get a TTY on both. But as soon as I run 'modprobe -v radeon modeset=1' the second monitor is switched off to black.