The Birthday of the Dog(1975)

Original title: El cumpleaños del perro

Two men who kill their wives so they can be together. A tailor goes from having his dog as his best friend to a young man, in a friendship that becomes so very close that the younger man murders his wife. He is helped by the other man. The husband wants to help him, but his wife wants to turn him in to the police. So he also kills his wife.