Identical twins Matthew and Gunnar Nelson and Carnie and Wendy Wilson have joined forces to record This Christmas, and the Nelson boys can't believe it took the quartet so long to work together.

"When we were growing up, the girls were just one hill over from us in the Hollywood hills," 49-year-old Gunnar tells Billboard.com.

The group shot the video for their new song at the home of Marilyn Wilson-Rutherford, the Wilson sisters' mother.

Gunnar tells the outlet the collaboration came together when a separate holiday project the Wilsons were considering fell apart.

"We did a lot of things (together) when our parents actually hung out together a little bit (in the 1960s)," he adds. "I think we were all in swimming class together when we were babies. The next time I saw Wendy and Carnie, Matthew and I were presenting them with their very first Billboard Award (in 1990). Then we stayed in touch after that."

The Nelsons first recorded This Christmas with their co-writer Alyssa Bonagura for an album of the same name last year (16).

"I’m very proud of what we were able to do last year with the song with our independent label," Gunnar says. "The finished performance was technically perfect, but I always felt there was more emotion to be had with the song. Honestly, when I wrote the song with Matt, I always envisioned the two girls doing this with us. These girls sound so honest and so vulnerable and so right for this song."