Parting asunder (diamerizomenai). Present middle (or passive) participle of diamerizō, old verb, to cleave asunder, to cut in pieces as a butcher does meat (aorist passive in Luk 11:17.). So middle here would mean, parting themselves asunder or distributing themselves. The passive voice would be “being distributed.” The middle is probably correct and means that “the fire-like appearance presented itself at first, as it were, in a single body, and then suddenly parted in this direction and that; so that a portion of it rested on each of those present” (Hackett). The idea is not that each tongue was cloven, but each separate tongue looked like fire, not real fire, but looking like (hōsei, as if) fire. The audible sign is followed by a visible one (Knowling). “Fire had always been, with the Jews, the symbol of the Divine presence (cf. Exo 3:2; Deu 5:4). No symbol could be more fitting to express the Spirit’s purifying energy and refining energy” (Furneaux). The Baptist had predicted a baptizing by the Messiah in the Holy Spirit and in fire (Mat 3:11).
It sat (ekathisen). Singular verb here, though plural ōpthēsan with tongues (glōssai). A tongue that looked like fire sat upon each one.