Two; having two.
[Latin duo, two.]
A duet, a performance by two people, and dual, consisting of two parts, both derive from this Latin source, the former via Italian duetto. A duologue (Greek logos, word) is a play or part of a play with speaking roles for only two actors; a duopoly (Greek pōlein, sell) is a situation in which two suppliers dominate the market for a commodity or service.
Some related words are based on Latin duplex, duplic-, from duo plus plicare, to fold; examples include duplicate, and duplicity, deceitfulness or double-dealing, as well as duplex itself, something having two parts.