If I recall correctly, you pay full price for the second seat but you don't pay all of the taxes.

In case you are on a flight that is overbooked, be sure to be firm. I have a friend who is quite large and purchased two tickets then booked two adjacent seats. It was with USAirways. On one of the flights the GA tried more than once to seat someone in the seat next to him. He might have caved in but his (adult) son was with him who insisted that he paid for that seat and it was his. To think that a GA would even try such a tactic is beyond me!