Definitions of dual:

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

  Example: "Ancient Greek had the dual form but it has merged with the plural form in modern Greek"

• adjective:   consisting of or involving two parts or components usually in pairs

  Example: "Dual controls for pilot and copilot"

• adjective:   having more than one decidedly dissimilar aspects or qualities

  Example: "A double (or dual) role for an actor"