George Seto, You're right that "You're Just In Love" is really one "song"--i.e., one composition, which is a duet with two different melodies and two different sets of lyrics. They are first sung separately and then simultaneously.

This type of two-part counter-melody song is not unusual in Broadway musicals. E.g., in "The Music Man" the two songs "Lida Rose" and "Dream [something]" are juxtaposed this way. And don't they do something like this in West Side Story? I seem to recall the song "Tonight" being sung between Tony and Maria while Anita and the gang guys sing "Stick To Your Own Kind" or some other song.

"Anything You Can Do" is a duet but it does not have two melodies or two sets of lyrics that are sung simultaneously.

I'm particularly interested, though, in songs that were not written together--probably not even written by the same songwriter or maybe even in not in the same era --which can be sung as countermelodies [partner songs].

Genie