I have a Radeon as well. 8.1. Acer monitor and dual link DVI.
If I'm Steam streaming to my living room and manually press the power button 'off' on my host monitor, the screens goes black on the Steam client. My monitor is powered off at home right now and if I remote in with VNC from work, it's a black screen and I can't access my desktop. When I shut my secondary monitor off, it automatically adjusts my desktop to fit one monitor, so it's clearly recognizing when these are powered off. I just wish I could find a way to manually start or stop that feature.