After upgrading to Sierra and Parallels 12 last week, I encountered the same issue this weekend while attempting to use Powerpoint in Parallels. After upgrading to Parallels v12, Windows 10 (under P.v12) would no longer detect the projector (extended display). After a little trial and error, I have discovered that if I turn ON "displays have separate spaces" in System Preferences -> Mission Control, and reboot Sierra, then Parallels v12 will detect and use the extended display.
So far, the only downside I have discovered by doing this is .... using the 4-finger swipe to change between workspaces does not change all screens in unison, I have to 4-finger swipe on each display individually if I want to change the workspace on a specific display. For the meantime, I can live with that minor inconvenience in order to restore the functionality to be able to present to a second display in Parallels 12.
Last edited: Sep 27, 2016