Duo | Webster's Dictionary of the English Language

duo
dŭŏ, ae, ŏ (acc. masc. duo as freq. as duos; cf. ambo. Passages with duo, Att. ap. Charis. p. 101 P....

A New Latin Dictionary by Charlton T. Lewis Ph.D. and Charles Short, LL. D.
duo
duo (1) ae, o (gen.duōrum, duārum, often duūm; accmasc. duos and duo), num.DVA-, two : pocula: men...

An Elementary Latin Dictionary