You can't do that with some airlines though since random seats are allocated at check-in, not when you book.

It seems like an obvious loophole I would have expected other airlines to have spotted?

I thought that all travelling couples did this. I compare it with being allowed to change the door that you can open on a TV show. The costs of choosing two side-by-side seats are greater than choosing just one seat, except for the extra room row. It also depends a bit on the length of the flight. Longer flights make you pay more for choosing a seat.

So... you pay for two seats every time you fly?

Edit: Oh, it's a trick for couples. Very smart.