It’s actually quite simple, only one server is needed. All the others can be players (the server can also run a player at the same time if needed)
Every Pi has to have its own playername (can be changed in the settings / reboot tab.)
e.g.
max2playserver for the server
max2playroom1 for the first player
max2playroom2 for the second player
Just make sure to choose a different name for each Pi.
Once the server and all the players are up and running, go to the webinterface of the logitech media server.
In the top right corner, there is a drop-down menu from where you can select each player.
Thats it, all players are running with the same server.
You can also choose to synchronise 2 players to play always at the same time.