twinship

noun

1. (uncountable)The condition of being a twin

   “1858 Charles Arad Joy et. al. - Addresses of the Newly-appointed Professors of Columbia College In this twinship of right and duty lies the embryonic genesis of liberty . . .”

2. Either of a pair of ships of exactly the same design

   “The Royal George, a twinship, was nearly as much approved of; and these two ships, by their fast sailing, were the principal, causes of the defeat of the enemy's fleets . . .”