It says that it is not possible to book an extra seat in the Premium Economy and Business cabins https://wwws.airfrance.de/en/information/passagers/forte-corpulence, in Economy it's ok. But it can changes according to different companies > Book two seats Please contact our Saphir service so that they can > assess your situation, check if a seat belt extension is necessary, > and complete your booking with you. > > If you are traveling in the Economy cabin on one of our long-haul > flights, and the width of the seat is not sufficient for you, Saphir > will offer you a second seat next to yours. This second seat will > provide you with greater comfort, while ensuring your safety with a > seat belt extension. > > ... > > Please note: in the Premium Economy and Business cabins, it is not > possible to book an extra seat with a seat belt extension, due to the > seat configuration and static armrests. > > In the Economy cabin: passengers cannot book two seats side by side > and for the same person using our online booking system.