If the fact that Glinda sings the alto part and Elphaba sings the soprano part in for good (because they’ve both taken from each other) in For Good doesn’t make you tear up a bit then I don’t know what does.
The only time in the play. Every other time they sing different notes, Glinda gets the high part, all dancing superficial sunshine, while Elphaba gets the deep supporting shadows; in “For Good”, Elphaba gives value to brightness and Glinda value to depth, because they do believe they have been changed for the better in knowing each other.
(Also note how through the vast majority of “Loathing” they share the same note, because neither can give up her hold on the starring melody, but “For Good” they can sing different melodies and words in perfect harmony and balance because they trust each other.)