As long as you cut power to the "chalupa" you probably wont damage anything. If you built some sort of clutch (or way to disengage the chalupa from the drill motor) you would be even better.
As far as i know if you have the chalupa and drill coupled running at different speeds (after reduction) they will fight, get very hot, trip breakers, and release their magic smoke rendering them useless. I believe a electic motor is similar to, say, the engine in your car. You can shift into first going down a hill and the motor will provide braking force (though this is just an analogy, as a gas engine does this because of internal compression and a electic motor does this because of... well im not entierly sure, my guess would be timeing on the commutator).
I'm by no means an expert and wouldn't be suprised if someone told me im totally wrong, but to the best of my knowledge what I said is correct
I suggest doing some research yourself.
Greg