Dos que duermen en el mismo colchón se vuelven de la misma opinión
 

In English?

Politics makes strange bed-fellows.
(I'm not sure that this saying is a direct translation)

I don't think it means the same thing at all. Robin's phrase seems closer to "Birds of a feather flock together", but even that's not quite right. "Dos que ... se vuelven" is talking about their opinions rubbing off on each other, or possibly about the emergence of groupthink.

It means that after being married for so long with a person, you end up thinking like him/her, or having the same faults.

Ah, I agree with the sentiment, but can't personally think of a similar idiom in English.

Buenas traducciones!:)

Birds of a feather .............. Es Dio slos cría y ellos se juntan, that´s to say that they were similar and got together, however, dos que duermen......is just the opposite, they were different and after rubbing elbows they become similar.