Ha ha, usually they say that dogs and their masters look more alike as time passes by. Never noticed it with couples.

I think people who are in long term relationships unconsciously mimic each other. The way they dress, talk, body language are some characteristics that are copied from each other. Correct me if I'm wrong, this is the theory of emotional face mirroring,i guess.

I heard a theory that people choose a partner who has a similar appearance.

When they are a good couple and spend a lot of time together, they start to behave similarly in some situations. They probably prefer to wear clothes that match each other (at least they don't belong to completely different styles). They often eat similar food when they eat the meals together and they have often similar ways of spending free time - what results (unfortunately... for many elderly couples) in developing similar overweight... Ok. Let's be more optimistic - they both are equally fit because of their sporty living habits  ;)

I have seen at least 3 married couples who look alike!