I know you can buy 2 seats, but you should also notify AA not to separate the seats. With flight/schedule changes etc, the last thing you want is to have the seating split across the plane.

On a separate note, I understand the need to be comfortable, however, numerous people fly long distance in coach and nothing has happened. Me personally, I wouldnt get the extra seat simply because there is no guarantee that I would have it. Might as well do one booking and get an aisle seat. Who know, maybe the flight is not full and you might end up with an empty seat anyway.