One person singing and others joining in as a chorus; to sing in response to another person's voice; to harmonize.

To reply to a friend's poem with one written in the same rhythm or rhyme; to respond to another's verse with a poem of one's own.

To act in coordination or echo one another; to support or cooperate closely with another person's actions or words.

(literary) Harmonious and in tune; often used to describe a deeply affectionate and stable relationship between a husband and wife.