I often travel with a friend who, when we're not flying first or business class, buys a second seat so she has somewhere to put her legs and generally stretch out a bit. She's done it on NW, AF and KL. They have all insisted that she will only get miles for one seat. The second seat is cheaper to buy, though: some of the taxes and fees are assessed per passenger rather than per seat, and she only has to pay those once.