Any updates regarding this? Does MultiplayerSynchronizer work between 2 clients?
EDIT: So MultiplayerSynchronizer syncs from authority to all connected peers, but with a common host/client architecture (like the one from the Godot networking tutorial) the only peer of each client is the server, they are not connected to each other and thus will not send messages to other clients. But you can either just set multiplayer.server_relay to true to enable the server to forward messages to other clients. Or I also heard that one can use WebSocket/WebRTC/mesh network or something to connect all clients with each other, and then it works as well, though I haven't tested the second approach yet.