In an ancient scholarly compendium of writings on Dionysos, Diodorus Siculus wrote: “He seems to be dual in form because there are two Dionysoi: the bearded Dionysos of the old times, since the ancients wore beards, and the younger, beautiful and exuberant Dionysos, a youth.”
Before him Dionysos (with kantharos) and a woman seated facing r.; a seated Athena and a second Dionysos with kantharos, looking round at them. Cf. Athens, NM 9687 (ABV 491, 58) with a seated H., two Dionysoi and a satyr.
2011, H[enk] S. Versnel, Coping with the Gods: Wayward Readings in Greek Theology (Religions in the Graeco-Roman World; 173), Leiden, Boston, Mass.: Brill, →ISBN, page 96:
The Athenians, for their part, decided to adopt this god as a κηδεστής, “a relative/son in law.” Comparably, one of the two Dionysoi in the city of Heraea in Arcadia was called polites. Gods as honorary citizens and owners of land and house, that is the ultimate expression of local inclusion in the world of ‘ours’—a “naturalization” in the words of Detienne.