Two or more Pi - how to?

I am new to this whole Pi streaming thing but have an IT background. This is long but I am trying to cover all the questions that may come up ahead of time.

Ultimate Goal:
To have have all stereos playing the same music at the same time (control mutiple systems) - be it spotify (dont judge me) or from our music library. Thus all rooms playing the same stuff during a party or the holidays when the family is over.

All of my equipment is solidstate, no surround, no digital inputs, etc.

Basic setup:

  • FreeNAS running Emby (Just to try it out) and a Samba share of the music directory.
  • Spotify premium family plan - I know its not lossless and I’ll cross that bridge later. My wife and I do not share our playlists etc with each other. She has hers, I have mine.
  • We have multiple stereo systems in the house, I am currently working with just two of them. I have one Pi with an Allo DAC on the good stereo and a second Pi, using the onboard headphone jack (will upgrade later), on the a second stereo.

I have tried a few different images while learning about this. At first Volumio on one and Dietpi on the other. I setup Dietpi on both and thats where the trouble begins.

The basic configs of the two systems are essentially the same.

Both have:
MPD: music player daemon
Gmediarender: DLNA audio render/endpoint
Raspotify: Spotify connect client
NetData: real-time performance monitoring
LLAP: Lighttpd

One has 128gb memory card and Emby installed on it too (again, just playing and seeing how things work).

Both systems default names have been changed
Pi1 = LivingRoom
Pi2 = FamilyRoom

Issue 1:
The Spotify application finds boths systems and allows me to play to one or the other via Spotify Connect, not to both.

  • raspotify (LivingRoom)
  • raspotify (FamilyRoom)

Issue 2: (and the bigger of the two)
Using and app named “Hi-Fi Cast” from my tablet I can only see one of the two systems.

  • DietPi (Ivo Clarys…) ← this is the livingroom system with 128gb card and emby running.

The app does not see the other system.

I am agnostic in regard to the applications and images currently running. Basically, I am new to this and the only reason I am running some of these things is because it worked - I have no idea what applications/packages function better than others.

How far off base am I?
What apps should I use, our hardware on hand consists of android tablet, ipads, mac laptops, iphones?
I really dont care if I have to wipe the Pi’s and start from scratch.