I'd just ask politely. In F, I think a window vs. aisle isn't that big a deal, especially on a short flight where you aren't going to be heading to the restroom that often. If you just say politely "Excuse me, would you mind swapping seats with my wife (or me) so that we could sit together, I think you have a great shot. You can also ask at the gate (you never know who will be a no-show and whose seat you can grab before it's given away). And if you say it JUST loud enough that other people can hear, maybe someone else will volunteer.