They can take turns, me thinks.

While walking the dog I've been thinking about this in a mathematical way. I think as a rule of thumb 3/4 of the volume should be melody and counter melody. That leaves 1/4 for chords amd percussion.

I am aware that as a cac handed melodeon player it would be very easy for the person on my right never to hear the tune - just my chords, so unless I'm leading the tune I tend not to play them. One melodoen out of 4 is enough on the chords and none if there is a keyboard, accordian or base playing - and these can take turns. Listening to a tune is as much part of a session as playing.

No percussion at all during the verses of a song and PP for melody instruments joining in, as a rule.
FloraG