Premex, I hope your generosity was genuinely appreciated by that couple.

In general, I am against asking anyone to change seats unless it's for a very good reason (so a child can sit with a parent, for instance; honeymooners, probably too). I think the best way to have someone change seats is refrain from asking right away. Wait till you get a sense of who you're sitting next to before you ask. If they're the type that acknowledge your presence, make small talk, etc., it helps. By casually mentioning your companion and the unfortunate circumstances that are preventing the two of you from sitting together, you stand a better chance than if the two of you approach the person during boarding.