236No motor vehicle, other than a motor vehicle in which there is a person licensed to operate a motor vehicle on a highway, trailer or other object or device shall be drawn by a motor vehicle on a highway unless there are two separate means of attachment so constructed and attached that the failure of one such means will not permit the motor vehicle, trailer, object or device being drawn to become detached, but this section does not apply to a trailer so designed and used that part of its own mass and of its load rests upon or is carried by another vehicle.