Try a PZM microphone for the left hand. Pressure zone microphones are very cheap but have a sound quality that is way beyond their price. They are great for instruments that move!! They must be mounted on a flat surface to work properly.

Put the microphone on a piece of half inch thick plywood about 8 ins square.

Put it on the piece of timber, on a boom stand, with the flat side of the timber facing the arc that the melodeons left side swings through.

It looks a bit weird on stage but you could decorate the plywood
with perhaps your bands name.

A preamp helps further.

I'd be interested to know if this works for you. PM me if you try it.

MofA