A short course in the DUAL (esp. Plato Ap. 20a-b):

1st decl.                                   2nd decl.                       3rd decl

-α      (nom/acc)                      -ω                                  -ε

-αιν  (gen/dat)                        -οιν                                -οιν

Definite article (all genders)=  τώ... τοῖν

Verbs: primary, 2nd & 3rd person = -τον  (act.) -σθον (mid./pass.)

  as in ἐστόν, 'they two are'

secondary (esp. aorist and optative) = -τον / -σθον   for 2nd person

-την, -σθην for 3rd person.  as in ἐγενέσθην,  'they two were'