Reserve two seats on an airplane and you are likely to get an entire row.
Here’s what to do: when you and a friend are planning to fly together, book one seat on the aisle and one at the window–in the same row. If the flight’s not booked, chances are slim that a fellow passenger will choose a center seat, between the aisle and window. So, you may well end up with three seats all to yourselves! If not, you can always offer to give up an aisle seat or window seat to the third party, and the two of you will still get to sit together.