Star Trek Easter Egg: Spock plus Chapel equals Harmony (an eerie combination)

Turns out Spock and Nurse Chapel aren't singing the same song (with different lyrics) like you thought. Spock is actually singing in harmony with Chapel. Here's what it sounds like when the two songs are synced up so you can appreciate the harmony. The preponderance of minor 3rd harmonies makes for an eerie and somber feel when the songs are combined.
The writers of this show are amazing, and they put this in the musical episode for us to find! Such tragic juxtaposition, the way the "jitterbug dance break" is timed perfectly to make Spock's final verse a solo, and then when he admits defeat, Chapel goes on singing about how excited she is about the future. She's ready! This is what life is really like.