Comment # 15 on bug 64850 from Jakob Nixdorf

Nothing of the things you suggested worked.

Without xorg.conf the problem is still present.

Booting with only DVI-0 plugged in works fine.
If I plug in HDMI-0 with X11 running everything stays the same and the HDMI
monitor says "No signal", when I try to activate it via xrandr both monitors
switch to black.
If I plug in HDMI-0 in TTY the TTY is switched to the HDMI monitor and the DVI
monitor goes black.