Apparently the duo was considered part of the family.
We need mention only the chief of them - the Collectio Anselmo dedicata, by an unknown author of the end of the 9th century; the Libri duo de synodalibus causis et disciplinis ecclesiasticis, 3 compiled about 906 by Regino, abbot of Pram, and dedicated to Hatto of Mainz, relatively a very original treatise; the enormous compilation surchard.
To this work were added two treatises, entitled Tractatus duo de speciebus et magnitudine figurarum curvilinearum, the one bearing the title Tractatus de Quadratura Curvarum, and the other Enumeratio linearum tertii ordinis.
Capitolinus, Maximini duo; Herodian vi.
Have the usual meaning (Duo = u l - uo, L 2 uo = Du 1 - Auo, .