It sounds like your problem is you hard coded a single room name in your game and all game instances end up in that room? So what your asking for is a method for detecting when X number of players have joined and place them in their own room so they don't interfere with other player games? You could create an dictionary of lets say 100 room names. when a player connects they always try connecting to the first one.. if it comes back full move on to then next. You specify how many peers per room you want.
I think you misunderstand the functionality of 'multiple mulitplayer objects'.
Imagine a game with 1000 players playing at the same time. Obviously we cant have them all on the same layout or 'area' so instead we split the areas up at host them on separate tabs for example. But since each area is its own separate host, how can the players from one area communicate with another? You could use a websocket server for chat i suppose. OR you enable construct 2 to let people have multiple 'multiplayer' objects.
This allows Player Bob to connect to area 1 as a peer with multiplayer.connect. Then bob also joins the chat host at multiplayer2.connect.
This allows cross 'realm' or 'guild' chat. This also increases the functionality of the multiplayer feature, you could for example have a 'host' hold all the data about what items the player is holding, his/her xp, current quests, etc etc.
You then have your 'main' server not only be a host to connected players, but also connect as a peer to the 'info/stats' server.
, someone has pm'd me and they are working on it.