20. When one or more vehicles of any kind are attached to a mechanically propelled motor vehicle either in front or behind the identification mark required to be exhibited at the front or at the back of the mechanically propelled vehicle, a duplicate of the identification mark shall be exhibited at the front or at the back of the foremost or rearmost vehicle attached as the case requires in the same manner as the identification mark is required to be fixed upon the motor vehicle drawing or propelling the vehicle.