"Like all doves, pigeons lay two eggs at a time, and thereby hangs the tale of "pigeon pair." Legend has it that pigeons brood always consist of one male chick and one female chick, which then grow up to mate and live together for life. (This doesn't actually happen ofcourse). But as a invocation of symmetry and balance in nature, "pigeon pair" has been applied to human babies since the mid-19th century. The primary meaning of "pigeon pair" is a boy and a girl who are twins, but an extended sense encompasses [our] situation, a boy and girl born a bit apart but constituting (at least for the moment) the only children in the family."
glad i now know where this comes from! Thanks Louise E for letting me know!