I've got a box I'm constructing, and put 1/8" birch play on the back, and 1/8" mahogany ply on the front (because I only had the birch when I started).  The mahogany has some nice deep tone, and the birch has more highs.

I'm contemplating putting a piezo under the top, as normal, but then perhaps placing another pieze somewhere else to capture the highs, and then mixing the two.

I figure if I put the second piezo on the inside of the back birch soundboard, I'll end up picking up knocks and scrapes from the instrument hitting against my body.

Another option might be to place the second piezo on the neck inside, figuring there'll be less lows on there because of the thickness of the wood.  

Another option might be to place it on the 1/4" sides, though might also get the scuffling noises there too, though not at bad as the back.

Yet another option would be to place the second piezo up in one corner of the top, where things aren't as flexible, and highs seem to be a bit more present.

If anyone has done something similar, any feedback would be great.  

Also, how would I wire this, to be able to variably mix the two?