Darby and Joan; old couple living in harmony
A set phrase referring to an elderly couple who live together in affectionate harmony, like the legendary couple from the Noh play 'Takasago'. Often used to describe a happily married old pair.
That old couple is just like Darby and Joan.
From the Noh play 'Takasago', where an old man (尉) and old woman (姥) appear as spirits of a pine tree, symbolizing a long and harmonious marriage.