UNIVERSAL CITY, California -- Both said it could happen someday, and now it has: Jessica and Ashlee Simpson have recorded a duet.

The sisters collaborated on a version of "Little Drummer Boy" for an upcoming holiday record from Jessica.

"It was a good opportunity because it's for a Christmas album, and Christmas is about family," Ashlee said backstage at the Teen Choice Awards taping Sunday (see [article id="1490028"]"Usher And Lindsay Win Big, Ashlee Burps, Britney A No-Show At Teen Choice Awards"[/article]).

Jessica just started the record, so the title and release date have yet to be determined, according to her spokesperson. It is known the singer will duet with another family member on the release -- husband Nick Lachey -- although the spokesperson was unsure which song the reality-TV stars will do.

Ashlee, meanwhile, is preparing to release her second single and video, "Shadow" (see [article id="1490045"]"Blondes Have More Fun In Ashlee Simpson's New Video"[/article]), and is talking with MTV about shooting a second season of "The Ashlee Simpson Show." "I'm pretty sure we'll do it," she said.

The reality show will be Ashlee's only TV series now that she has left the cast of "7th Heaven" in order to tour behind her album.

" '7th Heaven' is amazing, all the cast is so nice," Ashlee said. "But I'm happy to move on. [Artists] have stepping stones and then we do other things."

Ashlee's album, Autobiography (see [article id="1490098"]"Ashlee Simpson Retakes Top Spot On Albums Chart"[/article]), debuted at the top of the sales chart three weeks ago, surprising even Ashlee herself.

"Oh my God, I had no clue I was going to have the #1 record, so it was pretty darn exciting for me," she said. "I thought my first week I would sell 80,000, and the first day I found out I sold 150,000. I was like, 'What in the world is happening to me?' "

To celebrate, Ashlee's father and manager, Joe Simpson, surprised her with a trip to Cabo San Lucas. "I hadn't had a day off in so long, so ... [we] relaxed for three days," she said.

For an in-depth interview with Ashlee, check out [article id="1489216"]"Ashlee Simpson: The Rock Sister."[/article]