dual

consisting of or involving two parts or components usually in pairs

• “an egg with a double yolk”
• “a double (binary) star”
• “double doors”
• “dual controls for pilot and copilot”
• “duple (or double) time consists of two (or a multiple of two) beats to a measure”

having more than one decidedly dissimilar aspects or qualities

• “a double (or dual) role for an actor”
• “the office of a clergyman is twofold; public preaching and private influence” R.W.Emerson
• “every episode has its double and treble meaning” Frederick Harrison

a grammatical number category referring to two items or units as opposed to one item (singular) or more than two items (plural)

• “ancient Greek had the dual form but it has merged with the plural form in modern Greek”