>
>I realize this is a bad idea.  The mixers shown in the diagram are
>actually in separate, identical boxes.  If this circuit was in the
>design stage, we'd provide proper mixer-out and mixer-in connections
>on each box for the occasions where the user wants to connect two boxes
>together. Unfortunately, the inexperienced designer thought he could
>bring the summing node of the mixer to a connector and could connect
>the summing nodes from both boxes together to mix all four audios in
>both boxes.

A definite no-no.
Can you not just add a third box and mix down the first two?

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