The blurb says:
Once the bumper is removed, if your vehicle has a metal crash protection bar and the antenna will be within 3 cm's of this metal bar (once the bumper is re-fitted) you can install a 2nd Antenna (included with all PD1 sensors) to prevent any reduction in detection range or insulate the metal crash bar. This is only required on a small number of vehicles but the option is available if required.
Just struck me a as a good idea, if they work OK