@flick
I think there is a sort of bad solution.
The sound level watcher listen only ch 1 & 2. so if you set your second player on 3&4 it won't have any influence on the Sound level watcher. The general sound output (bottom) is set on blackhole 16 ch. So you can choose the channels in every player. You have also to set the capture setting input to blackhole. To ear the sound you must use two AUsoundinput set to blackhole ch 1&2 for one and 3&4 for the other. Then mix the audio and send to your output device. Not very elegant but I didn't find another way.
Best regards,
Jean-François