The duet speaks of the infinite but tragic like involving Chang'e and Houyi, that are dependant on the Chinese fantasy on the moon goddess, on which the movie is based on. Between several artists who have recorded variations of "Always and Forever" is definitely the American R&B vocal team Whistle, https://alwaysandforever19642.alltdesign.com/an-unbiased-view-of-always-and-forever-mine-56002132