It is known that the Greek language, at least in many of its dialects [not in Koine / modern Greek], kept for a long time the dual number along with the singular and the plural. Dual appears as a survival of those “specific numbers”, which in “protogenic” languages make the transition from singular to plural. … Opposition of the one to the many presupposes developed enough abilities of abstract thinking. It is indeed observed that the dual number vanishes when a society is developed spiritually… It is not a coincidence that it remained in the slavic languages only in the speaking of peasants, the slovenic language.
Ionic Greek, spoken in commercial cities that were in touch with foreign places, does not give any trace for the use of dual number (as much in Herodotus as in the inscriptions). Arcadian – mountainous and conservative – opposes in this the cypriote, an island-dialect. Agrarian dialects, as is the Thessalic and the Boeotian keep among the Aeolic dialects this number, which the philological languages of Asia Minor rejected. It is known with how much inconsistency the Aeolian and Ionian singers of Asia Minor used the dual number in the Homeric poems.
It is observed with a great surprise that dual was kept for a long time by that dialect, which together with the Ionian contributed the most to the development of the Greek thinking. The history of the dual number in the Attic dialect is connected with the political history of the city. The dual number, which was being kept as long as democracy was directed by the aristocrats, vanishes in the time of only a few years, when new social classes come to power. …
In Plato’s dialogues there exist a lot more examples of the “straight” clause δύο (two) with plural than with dual. As regards the δυοῖν, an “oblique” case, which in Aristophanes is always combined with a name in dual number, in Plato can be combined with a name in plural number, while, on the other hand, the type δυοῖν evolves to the type δυεῖν.