Isn't two front brakes a standard configuration on trikes, so not that unusual for the PBP bike check?
Of course, fitting an additional back brake may be worth it just for your stress levels.
In the UK recumbents are road legal with just one brake* but, of course, it's sensible to have more than one system on something which has such a high terminal velocity - hence putting the extra on the front, where it will actually do some good if I ever need to use it.
However in France, I understand each wheel much be equipped with a "braking device", on pain of an 11€ fine. Of course this does not apply to l'étranger during PBP**, but only legally. The PBP inspector can set whatever bar he wishes...
Anyway, this is all very OT and I apologise!
* because the seat height is less than 635mm (pedal cycles construction and use regulations 1983).
** as our bikes abroad are covered by the 1968 Vienna convention on road traffic, which mandates only that you must "have an efficient brake".