John Travolta & Olivia Newton-John

Has 3 songs in the following movies and television shows

You're the One That I Want
You're the One That I Want
Summer Nights
Summer Nights
The Flash - Poster
A drunk Caitlin pulls Barry onstage to do a karaoke song.
We Go Together
We Go Together
Grease - Poster
Grease1978
The very last scene, the cast is singing this all of them together at the School carnival.