Note the accordions made by Eric Martin:
http://perso.wanadoo.fr/accordeonsericmartin/page3.htm
You can get two rows -- with three voices each. If you're going to carry the weight of 2 keys in one accordion, you might as well be able to access them all without having to move 6 stops to change keys... plus you get all the basses and then some extras...
Nice looking boxes, too...

WHOA! Now those are different! Seems like if you're used to the finger positioning of the single row, the double would be an adjustment. Then again, I'm just comfortable with the single. Jeez! You'd be the talk of the town with one of those, though! Cool~!