Commentary Par VII 6

The neologism s'addua is problematic.  Readers are divided as to what exactly the double light represents, and there are widely various opinions.  Mazzoni (Mazz.1982.1), p. 142, suggests that these lights are, the one, earthly, the other, heavenly, that is, the emperor's past and present identities.  Bosco/Reggio (comm. to this verse) are of the opinion that the lights are of the warrior and of the legislator.  However, as we have seen in the preceding canto, Justinian seems most eager to put the military life behind him ([Par VI 25-27]); thus it would be strange for Dante to treat him in such wise.  And see Jacoff (Jaco.1985.1), pp. 323-24, arguing for Virgil's phrase 'geminas... flammas,' the description of Augustus at the helm during the battle of Actium, his brows casting a double flame, as he is portrayed on the shield of Aeneas ({Virg.Aen.VIII.680}, part of the same passage visited in the last canto: see C.Par.VI.79-81).  That seems a promising lead to follow.  However, it would probably be strange for Dante to have 'borrowed' Augustus's identity for Justinian.  There is also a possibility that Dante is thinking of the passage in Acts 2:3-4 in which the apostles are given the gift of glossolalia.  There appeared to them cloven tongues of fire; these settled on each of them; they were then filled with the Holy Spirit and began to speak such languages as the Spirit gave them to utter.  That is possibly reflected in what has been occurring in the opening lines of Canto VII; however, there may be a problem with the 'dispertitae linguae tanquam ignis,' which may not be all that well described by the phrase 'doppio lume.'

Bosco/Reggio point out that 's'addua' (inflected form of the verb coined by Dante, adduarsi) will soon be joined by such other numerical coinages as incinquarsi ([Par IX 40]) and intrearsi ([Par XIII 57]).