The twosome are indeed singing together, however, in a photo from Inside Llewyn Davis.
J.T. and the Great Gatsby actress star along Oscar Isaac (who plays the titular character) in the Joel and Ethan Coen-directed drama due out later this year.
The film tells the story of Llewyn, a singer-songwriter navigating the 1960s folk scene in New York City.
Oscar told Complex magazine earlier this year that Justin and Carey's husband, Marcus Mumford, "were a big part of recording to the music" for the movie's soundtrack.
Should be interesting to hear. Check out the teaser trailer below for a sneak peek!