It is not known if the division was territorial (as with the Merovingians) or purely a co-regency (as with the later princes of Benevento and Capua).
If that is the case, it is most probable that Tassilo ruled the diocese of Passau with his capital there at its see.
War broke out between the brothers soon after their father's death, but few details are known.
His existence is confirmed in the "Codex of Salzburg" (Salzburger Verbrüderungsbuch) where he is listed as unmarried, though some surmise that a certain Waldrada, mentioned as a wife of Theobald, was in fact Tassilo's.
On the other hand, he is identified as the husband of Imma (d. c. 750), by whom he had Grimoald and Swanachild.