Check out the web site of the air carrier that you will be flying. Most have seating diagrams for their aircraft. Pick out the best seats to fit your needs. When flying as a couple on airplanes with three seats to a row, book the aisle and window seats. This will reduce the likelihood of anyone booking the third seat.
If it does sell, ask the passenger if they would mind trading seats so the two of you can sit together. Most people won't have any objections.