Open up the amdcccle (AMD Control Center) as an admin or sudo amdcccle.
Change the display propertie there first to extend the desktop. Apply and try now to change the setting in display manager. If it doesn't work at first, reboot for the amdcccle changes, (I had to on one system and not the other)
Then you should be able to manage them regularly. Don't ask me why they pile on one more degree of difficulty to get performance out of our video cards.