I am currently working on building a tandem rotor (chinook type) helicopter for use as a photography platform. Not too far along yet, I have 2 x trex450 clones on the way from China at the moment. I have some braided carbon tubes from Dragon Plate that I was going to use to connect the 450 bodies. After some helpful words from Mr. Riopel, I have decided to use one motor mounted in the middle to power both rotors. I'm not certain, but I'm considering using the tail belts from two trex 600s instead of a shaft for the main drive train (mainly for sound considerations). I found a 400kv motor and a 13 tooth pulley that I think should give me the right rotor speed and torque.
I was also looking at getting the TH-2 channel mixer, and I was wondering if anyone has any experience with that module.
Looking forward to following your build. keep us posted.
I was also thinking, instead of using collective differential for my pitch control, just using the elevators already on the SP and switching the orientation of the 450 bodies to facing each other, instead of facing away.