MultiUser iTunes: iPhone + MacOS + AppleID

How can I use one Mac as the backup and AppleID server for two users?

Each user has a separate iPhone, MacOS login, iTunes library, and AppleID. The Mac is shared for reasons of backup (TimeMachine) and having a big screen to manage the library. Both libraries include Apple purchased content and other content from other sources, including some that are completely not recognized by Apple Music (personal recordings, rips from vinyl, rips from tape, etc.). The way I expected to use it was "log in as UserX, open UserX's iTunes library, do some users-itunesy stuffs."

I thought I had this working, but lately (possibly since Mojave+iOS 12.0) I'm getting:

"90 - 61 days ago" could be about when I first set up this two-user configuration.