There are two things: The "Library" (the mdb database and the files associated with it, which MusicBee creates/maintains to house information about your music files) and the "Music Files" (the actual music files mp3s, flacs, etc).
Each of these can be anywhere on your system. Your music files can be in many different places on your system and brought together in a single "LIbrary" (database+supporting files).
If you want one library that shows everything, though, rather than having separate "Dougie's Music" and "Clients' Music" and "All Music" libraries, your best bet would be to just create the "All Music" library and then create filters for "Dougie's Music" and "Clients' Music". If the music files for each subset are in separate locations, you could filter based on that. It doesn't need to be that way, though. You could create custom tag or repurpose an unused tag, tag each file for which subset it is in, and then use the appropriate filter set to this tag for when you want to see a subset. You'd use no filter when you want to see everything. You could even have all three available at once, each in its own pinned tab.
Maintaining separate Libraries is not easy and is not necessary with filters.
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