Titus Lenherr has shown that Graziano II's text of Duae sunt is closest to the Collection in Three Books and the Collection in Nine Books. None of the collections was the formal source.
The expanded text of Duae sunt is found in twelve different canonical collections.
Titus Lenherr, "Zur Überlieferung des Kapitels "Duae sunt," AKKR 168 (1999) 361-363.
Collection in 13 Books Vat. lat. 1361, fol. 103v