The disadvantage to using diodes in the system like Ed mentioned arises if your tow car has a body computer that monitors the tail/stop light bulbs for a defective or burned out bulb. If you incorporate a diode into the light circuit, this function can become inoperative, and may give false alarms on the dash panel.
Light monitoring systems are normally only installed on up scale luxury model cars. My first tow car in 2000 was a Saturn. It did not have a body computer with light monitoring capability, so I used diodes in the circuit. My next two tow cars were both Jeep Grand Cherokee Limited that were equipped with light monitoring systems, so I used a separate bulb for the tail/stop lights when towing.
The preferable lighting method will depend upon the lighting system of your tow car. If the diode configuration is compatible with the equipment that you ordered on your car, then it will give you the best performance. Otherwise you will have to install the extra bulbs.
Gerald