I have skimmed through LOST HORIZON twice now and have yet to find this supposed Kellerman-Kennedy reprise of "Living Together, Growing Together." There IS a reprise of the tune, right after the Fertility Dance (oy),but it's choral. Interestingly, the underscore in two Kennedy-Kellerman scenes is indeed "Living Together," and one scene (right before Kennedy's irrigation system is revealed) has a rather abrupt edit, so perhaps there was a reprise there.
Yes, these are the sorts of things I spend my day trying to figure out.