Can it be, that the Greek grammarians invented their dual number for the particular benefit of twins? – Herman Melville, Redburn. His First Voyage, 1849

My sister and I, you will recollect, were twins, and you know how subtle are the links which bind two souls which are so closely allied. – Arthur Conan Doyle, The Adventure of the Speckled Band

Twice as much to love, two blessings from above. – Author Unknown