Hello Rob, Donald Smith from Scotland here. I am trialing the Mac version of office and have come across the same problem with PowerPoint.
I have found the two menus as you say, but no method to switch on mirroring on either. The Microsoft help system explains something that is physically ‘not there’ in the menu structure.
How did you manage to resolve the issue in the end. I just want to show 2 full screen presentations on two screens.
Regards
Donald Smith