Apropos my post below about a key-switching 3-row melodeon, I would like to point out that they do already exist, as sold by Gabbanelli of Texas.
A few notes on Gabbanelli's two-tone:
1. The treble side is 2-voice MM in each key, as far as I know.
2. It features Gabbanelli's special look in celluloid.
3. It features the special feel and sound of Gabbanelli's reedwork.
4. I hope that someone will correct me if I'm wrong, but the bass/chords do not switch.
I have never tried a "two-tone" Gabbanelli, just about a dozen other Gabbanellis. I would love for Gabbanelli to send me a "two-tone" for review on this blog.