Say you had a provisional Class 1 (C+E) on you license, would you be allowed to drive on L plates if you had another driver with you like you can in a car?
Assuming you already hold CAT C and your proposed supervising driver has held CE for 3 years, it’s fine. Check insurance of course.
In fact, it’s exactly what you get with most trainers. The vast majority are not qualified, registered or trained themselves. So what’s the difference?
Pete 