I recall having read that in some scandinavian country, the bikes to teach how to ride -over here across the pond there are driving schools- have dual controls for learner and instructor, just as the cars. In the car I learned to drive with, the passenger had a dual set of pedals -no steering, he would just stretch his left arm and correct the trajectory with my steering wheel. When going to the DMV for the test, the instructor should have to go on the passenger seat, and the examiner in the back seat. It is mandatory that the dual pedals have a buzzer, so the examiner know when the instructor is using them to correct the person under exam, but it is not mandatory to have it connected all the time, so if the examiner doesn't ask the instructor to turn it on, he would assist the person under exam in case he needs it without it meaning to fail.