Ok So just as an update to the full screen issues everyone has been having on the external monitors i to had the same issues when i updated to sierra. This is how i fixed my issue and now am using 2 external monitors running a different VM in each with no trouble. First Go to your mission control setting on mac and make sure use displays in separate spaces is checked. Now Go to Dock settings and "The Prefer Tabs When Opening Documents" Scroll Bar Needs To be set to Manual. Now Open up Parallels Desktop Settings For your VM and make sure use all displays in full screen is not checked. After All changes Log out and back in to your Mac. Enjoy You should now be able to open your VM's and drag them to any external monitor then click the Green Full screen button for them to go full screen on that monitor.
