Paul, it is not absolutely necessary to use both, but saying that is with some reservations.
The 12N or Black socket takes care of all your day to day running lights, ie Brakes, Indicators, Side Lights, Tail Lights and Fog Light.
The 12S socket provides the other things, such as Reversing Lights and Fridge 12v supply whilst towing, and a 12v supply to charge the leisure battery in the caravan whilst towing.
It is rather silly to not make use of the advantages the 12S socket provides, so in reality, YES you do need both.