I do not have PowerDVD installed.
But because you mentioned about the demo mode it has, I looked in to the settings in Catalyst Control Center settings.
And sure enough, in video settings it does have a setting for enabling "Demo Mode", with split screen the preselected mode.
Turning that off fixed the problem immediately.
I have no idea how they could think of having it turned on by default. Was getting annoyed that I couldn't find the problem.
Thanks for the help, and I'll be sure to check CCC if it ever appears again.