In 9.2.0.z GDM gained fixes for PreferredDisplayServer=legacy-xorg to allow using a shared X server between the login screen and first user session.  

Unfortunately, those fixes were incomplete and don't allow using an additional shared X server to be the login screen and subsequent concurrent user session (as initiated with fast user switching).

Disney uses fast user switching in production and needs it to function with PreferredDisplayServer=legacy-xorg (edited)