When combing rooms with multiple microphones you may want to have them all in the same Gating Automatic Mixer or Gain Sharing Mixer based on combined state (Image 1). With a gating mixer this would allow for 1 NOM (number of open mics) for all microphones in use when rooms are combined. For a gain sharing it would mean all microphones adhere to the maximum gain in a combined state. Here is a simple way to accomplish this task by using audio routers and adjusting a couple of properties. Set the Type to Multi-channel and make sure the Selection Mode is set to Destination (image 2).
To set up the logic expose the Output #n select control pins on the routers( Image 3 and Image 4). Trigger those from the room combiner.