Comment # 5 on bug 1009932 from Ronny Peine

@Roger:

Could you please test the following:
Disable your screensaver. If you are using xscreensaver make sure to kill the
xscreensaver process, too, to reduce the possibility of influences to the test.
Start the following in a graphical terminal, this will force the display to the
standby DPMS state:
xset dpms force standby
Than after some seconds move your mouse. If the second monitor still keeps in
suspended state this bug is most likely in nouveau and the screensaver is ruled
out.

Kind regards,
Ronny