And since Ari's boyfriend, Big Sean, is featured on Justin's track, Ariana rapped his part.
They also performed Bieber's "All That Matters," off his Journals project.
And look at him dancing like crazy to Ari's "Love Me Harder"! He took fellow Canadian The Weeknd's place in the song.
And, of course, to top everything off. Here is Ari's brother, Frankie, who was at the Kids' Choice Awards, reacting to the whole thing.
Me too, Frankie. Me too.