I think it is important to point out to the OP that will need to exercise a certain amount of vigilance in defending their extra seat from poachers and either lazy or aggressive staff - for example, on an overbooked flight, the GA may try and strong-arm that extra seat away, or an FA may tell someone to sit there, ignoring the claim of the OP - sure, they will be compensated after the fact, but that is little comfort when you're about to sit for 14hrs on a long flight. Other passengers may covet the seat (less likely with a couple booking an empty middle), and you may need to get aggressive in keeping them away. Not a sure thing that trouble will happen, but the OP should still be aware and vigilant, at least until the flight is underway.