Comment # 14 on bug 64850 from Jakob Nixdorf

'xrandr --output DVI-0 --off' causes the first screen (HDMI-0) to turn black
too, and the DVI screen gets really switched off (monitor switches to standby).

Blind-typing 'xrandr --output DVI-0 --auto' turns the DVI screen back on (still
only a black screen) and the HDMI screen also stays black.