I have been dreaming of what kind of guitar I would like to build one day, and your double back matches one of my ideas almost exactly. I was even thinking of using spruce for one soundboard and cedar for the other. I was thinking of carving f-holes in the second soundboard, but it looks like you have a gap between the soundboard and the heel. That's a neater solution--you don't break up the surface of the soundboard much, and the second soundboard isn't damped by the heel.
I was also wondering about the risk of wolf-notes with two soundboards, but your music sample sounds clean.
The second idea I had was to connect the two soundboards with a soundpost below the bridge, as they use in violin construction. A direct connection between the bridge and the second soundboard should give it more energy, causing it to produce more sound.
Finally, I read that a 19th-century accoustician proposed that Stradivarius and del Gesu violins sound so good because the top and back have tap frequencies a half tone to a whole tone apart, with the back higher than the top. I thought it might be fun to try to get the tap tone for the second soundboard a semitone higher than that for the top, with the back a semitone higher still.
It's all very simple, as long as it's purely theoretical.