Not quite sure where I first heard this but I suspect the song was performed on a "Uta Kon" (うたコン) in the last few months of last year. This is another duet by actors Ken Yamauchi and Masako Izumi（山内賢・和泉雅子）titled "Hoshizora no Futari" (Starry Sky Couple) from March 1967.
About 6 months earlier, Yamauchi and Izumi had a huge hit with "Futari no Ginza"（二人の銀座）, a song that had been penned by The Ventures and Rokusuke Ei（永六輔）. This follow-up was written by Hisatoki Izumi（いずみひさとき）and composed by Makoto Kawaguchi（川口真）, and has a bit more of the sweet and a little less of the urgency. With the grand horns and chorus in there, I was thinking that "Hoshizora no Futari" had also been a theme song for a movie of the same title but didn't come across any indication of that in Yamauchi's J-Wiki article.
Yamauchi and Izumi still sound like that adorable teen couple hopelessly in love and not shy to show it. The video below has a shortened updated version that sounds as if The Ventures took a crack at arranging it.