Not really confusing at all. They combined the two with the verses taking their melody from 'Heart & Soul' and the 'boom de yada' from 'I Love the Mountains.'
Upvoted for recognizing that the melody is the same as in 'Heart & Soul' but the 'boom de yada' melody is also in 'Heart & Soul' as harmony. Well, it's technically not harmony (my music theory is a little shaky) but it's in the background of the melody. Just listen to the first few measures of 'Hear & Soul' and you'll hear it.
tl:dr All parts of the mountain song melody are in 'Heart and Soul.'
tl:dr All parts of the mountain song melody are in 'Heart and Soul.'