When booking two seats

Always book an aisle and a window. This way you have a better chance that someone is not going to take the middle seat and leave you with the extra space. If it is booked you can easily exchange one of your seats with the person to sit together. Also ideally book them towards the back of the plane, preferably a few rows up from the rear toilets. People typically take the seats closer to the front so if you’re near the back you’ll have the greatest chance of the extra seat being free!