The figure of speech being used is a pun, as "chord" is a musical term for a combination of notes played together, but it is also used here to mean harmony or agreement in a relationship. The deeper meaning is that the couple always worked well together and were in sync, like two notes played in harmony.
Overall, the pun adds a lighthearted and playful tone to the description of the couple's relationship, emphasizing the connection and rapport they shared.
