macOS Photos: Optimized Storage version and External Drive Full Resolution version of same library

If your full resolution iPhotos Library won't fit on your internal drive any more you can switch to optimized storage. Even for a large library that's only 30GB or so [1], But they you can't do true backups with full resolution images. I like to have 5-6 full res backups rotated amongst drivers and servers including Time Machine. The trick is to have two macOS user accounts with the same Apple ID…