@ayertay, ok, makes sense, sorry for being slow in understanding!
I see you've already started a discussion with Leonard on this:
I think the next steps are:
(1) add a bicopter to the built-in SITL support (in libraries/SITL). It would be very similar to this one:
but would take advantage of the support for tilting rotors. See the example of the tilt-tricopter frame here:
(2) add a AP_Motors_Bicopter motors backend. As Leonard suggests, starting with the existing AP_MotorsSingle.cpp and creating a AP_MotorsBicopter.cpp would be the way to go.
Then get it flying with ArduCopter in SITL. Once that works, then get it flying with ArduCopter in your real airframe. Only after that would you then add support for that airframe type in the quadplane code so you can transition.
I hope this helps!