PASADENA, California -- Jessica and Ashlee Simpson aren't the only sisters in pop poised to have a sizzling summer.

Hilary and Haylie Duff are touring together, recording albums and releasing a duet. The single, a cover of the Go-Go's' 1981 classic "Our Lips Are Sealed," will be featured on the soundtrack to Hilary's movie "A Cinderella Story," opening July 16.

"We really wanted to work together, and my label knew that, so we found this song and we're like, 'Yes! We have to do this!,' " Hilary explained excitedly backstage at Saturday's Wango Tango On-Air festival, where the sisters debuted their version. "I thought it fit in the movie because there's a secret relationship going on in 'A Cinderella Story' and it's kind of like a secretive song," Haylie said.

"The movie's kind of about what goes on in high schools and how people are mean to each other, so I think that it fits really good," Hilary added.

The Duffs recently shot a video for "Our Lips Are Sealed" in Toronto with director Chris Applebaum (Britney Spears, Fountains of Wayne). "The video is awesome," Hilary said. "We were in a car driving around the city and we were in a fountain, which was really fun, but it was freezing outside."

"We had the best time up there," Haylie said. "We had the whole band with us and our family, so it was a really cool experience."

Haylie, who wrote "Inner Strength" on Hilary's Metamorphosis (see [article id="1484672"]"Can't Get Enough Duff? Get Ready To Meet Haylie"[/article]), is currently helping her sister on the follow-up as well as recording her own album, which will likely feature a Hilary appearance. "We're halfway finished and it will be out in August," Haylie said. Hilary's album will likely follow a month or two later.

"I've recorded like four songs and we have almost all the material picked out," Hilary said. "I'm writing with Haylie a lot and I get to collaborate with the writers, so it's very personal and I love stuff that we have so far. And I don't think I've ever really said that before, like, I feel so cool about it. I'm really proud of it. I can't wait to record it and get it out there."

Hilary plans to work with the same stable of producers who manned Metamorphosis, including the Matrix (Avril Lavigne), John Shanks (Michelle Branch) and Meredith Brooks (Jennifer Love Hewitt). "If it ain't broke, why fix it, right?" Hilary said.

Haylie will be opening for Hilary on her summer tour, and the evenings will end with the "Our Lips Are Sealed" duet (see [article id="1486621"]"Hilary Duff's Idea Of Summer Vacation: Concerts, Album, Film, Concerts ..."[/article]).