The 37-year-old actress, who voiced Princess Anna in the 2013 movie, shared a video of herself performing to "Let It Go" as Menzel actually sang the song in the background.
The clip begins with a closeup of Bell lip syncing to the hit theme song. The camera pans to show Menzel, who starts laughing mid-rehearsal when she sees her co-star recording the video.
"SISTERS. (I [love] u @idinamenzel," Bell captioned the post.
Just Jared reported Bell and Menzel, who voiced Anna's sister, Queen Elsa, reunited to attend the Walt Disney Family Museum's 2017 Fundraising Gala. The pair performed songs from Frozen at the event, including "Let It Go" and "For the First Time in Forever."
Bell and Menzel will reprise Anna and Elsa in the sequel Frozen 2, which opens in theaters in November 2019. Bell said in October that she indulged one of her daughters by dressing as Elsa, not Anna, for Halloween.
"When your daughter demands you BOTH be ELSA for Halloween...you GRIN AND FORKING BEAR IT. #halloween," the actress wrote.