"We Both Know," the duet that Colbie Caillat and Gavin DeGraw have recorded for the soundtrack of the upcoming movie Safe Haven, has just hit iTunes.  The two artists wrote the song earlier this year, and first performed it publicly during their joint summer tour.

Talking about the song's creation, Colbie tells ABC News Radio that the studio "wanted a beautiful song for the closing [credits] of the movie," so she and Gavin got together at her house in California and went to work.

"We started writing the song, and he was playing the piano and he had this beautiful melody idea for the chorus," she recalls.  "And we just started, you know, shouting out ideas to each other."  The two finished up the song while they were on tour, and Colbie says she was thrilled at the reaction they got when they played it on stage together.

"The first time we played it was at the Greek [Theater] in L.A.," she tells ABC News Radio.  "And all of our fans just loved it and it, of course, went up on YouTube and stuff like that.  And it was nice because, you know, writing a new song, it's scary.  You never know the reaction [you're gonna get]."

Billboard reports that Colbie and Gavin will perform the song on TV together for the first time on NBC's Today show on New Year's Day.  The movie itself, which stars Josh Duhamel and Julianne Hough and is based on a Nicholas Sparks book, will be in theaters February 14.

Meanwhile, if you like "We Both Know" and you want to hear more of Colbie and Gavin singing together, check out their duet on "Baby It's Cold Outside," which appears on Colbie's current holiday album Christmas in the Sand.  She says that when it came time to record the song, she was adamant that Gavin be her duet partner.