Now what’s the use of alternating opposites? All ancient cultures were
obsessed by duality. Think about Cain and Abel, Yin and Yang, Remus
and Romulus, Chaos and Order, Newton and Leibniz, or if your cultural
background is quantumphysics: matter and anti-matter. All creation is
the product of two counteracting extremities. Joyce knew: If not for
human imagination, twins are unbelievably incredible.

AHII (1997)

1twin adj 1 Born with one other or as a pair at one birth (~ brothers, ~ sisters). 2a having or made up of 2 similar, related, or identical units or parts b being one of a pair, esp of officially associated towns (ME, fr OE twinn twofold, two by two; akin to ON tvinnr two by two, OE twa two)
2twin n 1 either of 2 offspring produced at 1 birth 2 either of 2 people or things closely related to or resembling each other 3 twin, twin crystal a compound crystal composed of 2 or more related crystals grown together in an orientated matter
3twin vb vt 1 to bring together in close association 2 to form into a twin crystal ~vi1 to become paired or closely associated 2 to give birth to twins 3 to grow as a twin crystal