Each instance is started separately by separate init script, and as jodal says above, each instance requires its own configuration files and dirs for some of the functions (implies we need to do this for cache_dir, data_dir, logging conf, playlists_dir, but mot media_dir bc it’s r/o).
I have dropped this task since I last wrote here, but will soon (next 2 weeks) get back to updating and testing my multi-mopidy-server configuration and will share it with you.
In the meantime, I’m curious to learn more about your setup. I read elsewhere that you are using 2 12-channel amps. I’m curious what amp you’re using.
I think you have more zones than I do, and are planning to use mopidy exclusively for all switching/routing (1 sound card per zone). If (when?) that takes you to a dead-end due to lack of mopidy support for dynamic output selection, might I recommend you consider using a professional audio dsp processor/mixer to separately perform switching/routing/mixing?
For example, I have 7 zones (rooms) in my condo, spanning 3 physical floors. My ubuntu server has 3 sound cards, 1 for each floor (group of zones) which is plenty since zones within a group are not really acoustically isolated. You’d never want to send different sources to different zones on the same floor, bc they bleed into each other and it wouldn’t make sense. I have 3 mpd/mopidy servers, one per floor/group. I use a Biamp Audia in between the ubuntu server and my B&K 12 channel amp. The 3 audio sources from the ubuntu server can be routed to any of the 12 speakers (and do EQ, delay, leveling, metering…) using the Audia’s interface.
The moral of the story is that I would encourage you to simplify wherever possible. Ask yourself whether you really need 12 independent sources for your house. The dsp/processor is of course optional, but if you’re a tinkerer, and have a few hundred bucks and some time to spare, you will get more power and flexibility using a dsp.
As for your questions about users… it is not clear to me what you are trying to accomplish. I’ll assume you’re not talking about permissions or security, and are simply talking about multiple clients accessing the servers. mpd (and mopidy) support concurrent connection to multiple clients. Each server binds to a distinct port, and your clients need to know which port to connect to for the audio source it is trying to control. I, e.g., use ports 6600-6602 for my mpd servers. 6600 is basement, 6601 is first floor, 6602 is 2nd floor.