So, you have two separate Libraries, on two separate MC Servers, and you have set up being able to run either as a Server, and either as a Client of the other Mac. Correct?
Does each Mac have a different set of media files on it, or accesses a different set of media files stored elsewhere, or something similar, for you to set them up this way? It is unusual, to say the least. If you don't have two separate sets of media files that you are deliberately keeping separate, please tell us more about where your media files are stored, how each Mac accesses them, and why you have set MC up this way.
Generally speaking, one PC/Mac is set up as the MC Server with access to the media files, and the other connects to it as a MC Client. Then either Mac could play any media to either Mac's speaker. Plus you can link Zones across the two and play music on both Macs at the same time.
Mac1 Zones can only be created on Mac1. Mac2 Zones can only be created on Mac2. If Mac1 is the Server, it will have a default Zone of "Player". On Mac2 running as a MC Client, that Zone will appear as "There" rather than as "Player". In fact, whatever Zone is currently selected on Mac1 will be presented as "There" on Mac2.
On Mac2, if you link the "Player" Zone to the "There" Zone, you can play to either Zone and music will play on both Mac1 and Mac2. You would then probably want to right-click on one of the Zones and select Adjust Link Timing to synchronise the playback in each zone. Otherwise they are likely to be a little out of sync. I still get the music go out of sync at each track change though, which is annoying, and why I don't use linking of Zones across PCs.
https://wiki.jriver.com/index.php/ZoneSync Plus search the forum for ZoneSync.
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There is also an alternate way to set this up, if both Mac1 and Mac2 are running DLNA servers. This is using
Zonesync 2.
Let's just assume that Mac1 and Mac2 are the names of the Macs as they appear on the network, then with each running their own local Library, the other will appear under Playing Now as a Dynamic (DLNA) Zone. So on Mac1, Mac2 will appear under Playing Now, and on Mac2, Mac1 will appear under Playing Now. You can then link Player on Mac1 to the Mac2 Dynamic Zone, and then play music on Mac1 and it will be played on Mac2 as well.
In this second configuration I find that the audio synchronisation is much better. There is no Adjust Link Timing function and DLNA commands are used to automatically retain synchronisation.
But then if you also link Player on Mac2 to the Mac1 Dynamic Zone, strange things can happen. Probably MC will end up unresponsive, as Mac2 is playing to itself and Mac1, which is playing to itself and Mac2. So don't do that.
However, now that you have linked Mac1's Player Zone to Mac2's Player Zone, playing anything on Mac2's Player Zone will also play on Mac1's Player zone. The display of track in Playing Now may look a bit different though, with Mac2's Player Zone only adding one track at a time, while Mac2's Playing Now shows all tracks added.
Just watch what happens on track change using the like this way around. I saw some strange things, such as tracks being skipped. I saw only every second track being played. If you see that, please report it.
You can see what is going on via "Playing Now > Overview". Both Macs should show what is in Playing Now for their local Zone.