we will suggest using an external mixer to balance the sound levels.
take the output from each pocket operator to a separate mixer channel.
but what about sync? either, you can use an external click-sync (mode SY2) routed to the input of every pocket operator.
if you prefer to chain together the pocket operators, you can put a "stereo splitter" cable in the output jack to get separate audio and sync. connect the sync signal to the next pocket operator, and audio signal to the external mixer.
if you are running out of mixer channels, you can let several pocket operators share a channel, by connecting them as a chain (with the usual sync modes). in that case, of course, you will have less control over the relative balance of those units.