Originally Posted by AtlanticX

It probably helps if we know what it's for.

I'm pretty sure it's possible: I read about cellists that will reserve an extra seat for their cello's, as they don't want to check these instruments and expose them.

Or more commonly, 'persons of size' will have to reserve an extra seat for their own comfort.

Do mind, if it's about getting cheap segments/miles, you will only get them once per person flying.

Originally Posted by FB T&C

2.7 The Member must actually travel to earn Level and Award Miles. Level and Award Miles will not be awarded on unused, forfeited, fraudulent, lost, out-of-date or refunded tickets. The Member can earn Level and Award Miles only for the flight flown, even if the Member purchases tickets for other people or purchases several seats on the same flight. As an exception, on Air France and KLM operated flights, a Member can earn Award Miles in case of booking of an extra seat (for a stretcher, for transportation of fragile oversized baggage in cabin, for overweight person…) provided the extra seat is booked under the same name as the Member and in the same booking class as the Member’s one.