Update: I ended up having to custom-roll a login screen for a second login to make this work. Seems if you set up the dummy account in both files, then execute a script in the UI file to force a second login with the user's real credentials, it actually works. Not ideal, but at least it'll prevent having to keep all the credentials in both files or (worse) going to a unified model.
So for future reference, make sure the account you want to use to log in with exists in all files. Then you can use scripting to force re-logins as needed.
Thank you for taking the time to describe your workaround/solution. I had been watching this thread in anticipation of learning something from whatever responses people offered.