Quote Originally Posted by giggsy
if the flights are full how can they buy 2 tickets?
According to the airline's Website on passengers requiring extra space (http://www.united.com/page/article/0,6722,52985,00.html), this policy concerns passengers who meet the following criteria on United or United Express flights:
* are unable to fit into a single seat in the ticketed cabin
* are unable to properly buckle the seat belt using a single seat belt extender
* are unable to put the seat's armrests down when seated
Those who fit under any of these criteria will be relocated next to an empty seat. If there are no unused seats available, then the persion must either purchase an upgrade to a cabin with available seats or change to the next available flight and purchase a second seat. Those who meet the criteria and who don't purchase the second seat will not be allowed on the flight.

Apparently Southwest Airlines in the USA has a similar policy.