Tolkien states that “a few of the commonest verbal notions are expressed by biconsonantal forms, though the verb form in Adunaic is usually triconsonantal” (SD/416). Despite this statement, there are quite a few attested biconsonantal verbs, though the majority are only attested in an agental-formation: bêl-, bith-, zîr- and more ambiguously mag- and bat-. Since the vowel of verb stem is often lengthened or fortified in such formations (SD/427), it isn’t clear in all cases what the vowel of the verb stem is.

The only unambiguous biconsonantal verbs with inflections are nakh-, yad-, and in the draft Adûnaic grammar khay- and nek- (an earlier form of nakh-). Based on these forms, my best guess for the conjugations of biconsonantal verbs is as follows:

There are no attested examples of the aorist form of a biconsonantal verb and the rules above are an educated guess based on formations for the triconsonantal-verb. There isn’t enough information on the continuative past to make even a wild guess as to its formation. See the entries for the individual verb tenses for further discussion.