Passengers who won’t comfortably fit into a single seat can buy a second, adjacent seat at a 25 per cent discount – with the promise that if any unoccupied seats remain in that cabin, the cost of the second seat is refunded.
You use the actual passenger’s name for the first seat; for the second, make the first name “Extra“ and the second name ”Comfort Seat”.
Make sure (obviously) you pre-pay for two seats together, then check in as normal and get two boarding passes, including one carrying the name “Extra Comfort Seat”.
Virgin Atlantic urges passengers with a high body mass to book extra space, and stresses “you will not pay any passenger taxes, fees, charges or surcharges on the extra seat”.
Air France also says: “You may be denied boarding for security reasons if: you have not booked a second seat, the flight is full, and you are unable to sit in a single seat.”.