Counterrotating twins don't stall in vmc demo before reaching vmc as long as the density altitude is high enough. Assymetrical thrust goes down due to the thinner air.

Even so, as an instructor though you have to think about what is going to happen if that one in a thousand student manages to kill a motor while setting up a stall and then follows through before you can push the nose down. I guess you will have to hope for the best and be alert. I am told the best policy for instructors is to assume the student is actually trying to kill the both of you lest it really happen, and it has unfortunately.