1. Swear To Shake It Up

Sam, Dean, Castiel, Gabriel, Michael, Lucifer, Garth, Balthazar, and a few others are all on the same Acapella team; The Treble Makers
Sam and Gabriel are an open couple but they want to hook up Dean and Cas (and Lucifer and Michael if they can)
So when auditions for solo part come up, Cas and Dean are already competing.

Hopefully they win their upcoming competitions! Especially when the Hell Hounds will soon be arriving at their school.