Doesn't work...a friend tried it...booked two seats via this deal.
when it got to checkin "where is the other person?".
"umm they are sick"
"ok then.... you forfeit the right to the allocated seat beside you"
Yes but my point was although you forfeited the right to have that seat reserved, you may still have that seat vacant as it's been reserved up til that time. So you may get lucky and still have the seat free.
I can understand airlines not letting you have an extra seat if not needed - airlines regularly overbook flights because they assume a percentage of no-shows. If their flight is overbooked and they don't get as many no-shows as they expect, they don't want to bump someone off the flight if there is a vacant seat.