While it is illegal according to the airlines, in practice they have a hard time catching it if the first name initial and last name is the same. The best "opportunity" occurs if one of the flyers is a xxxxx and xxxx, Jr. Simply using "xxxx" on the reservation would almost certainly allow either xxxx or xxxx, Jr to fly and xxxx receive the credit. Of course, like someone said above, you could always be caught on the way back by someone particularly vigilent.