Aly & AJ are still "in deep with 'Insomniatic'," according to older sister Alyson Renae Michalka, but the sibling pop duo has its third album already in its crosshairs.

Aly & AJ are still "in deep with 'Insomniatic'," according to older sister Alyson Renae Michalka, but the sibling pop duo has its third album already in its crosshairs.

In fact, Amanda Joy Michalka tells Billboard.com, "the whole new record is pretty much done being written." And once Aly & AJ finish a two-week run on Miley Cyrus' Best of Both Worlds Tour and filming separate movies, they plan to get back to working on it for release later this year or in 2009.

"We haven't really talked about the sound yet," AJ notes. "Aly and I are just throwing around ideas, writing cool stuff on piano, lots of different, cool things. I don't want to give too much away right now. It's kind of in the laboratory. But it's different. We're always wanting to reinvent ourselves and come up with something new."

Aly agrees, adding that "the third record is probably going to be even more different than ('Insomniatic') was from (their 2005 debut, 'Into the Rush'). We're still staying Aly & AJ and who we are, but at the same we're not doing the same thing over and over and over again. We want to surprise our fans."

The duo has just released a new single, "Like Whoa," which they hope will keep those fans sated while they head off to their film projects -- AJ to the Peter Jackson-directed "The Lovely Bones" in New Zealand, Aly to Austin, Texas, where she'll work on "Will," a drama that will also allow her to write and perform some music for the soundtrack. "We've always wanted to do stuff that's separate," Aly explains. "Music is where we come together and are able to share, but the acting is definitely separate."

They say they'll enjoy being together with Cyrus for an eight-show swing that starts tomorrow (Jan. 11) in Detroit. The duo met her at a 2005 Disney Channel gathering and became fast friends; they plan to sing Kiss' "Rock and Roll All Nite" together during the concerts and generally behave like "all girls, rockin' out on stage," according to Aj.

"The thing that brings us together is we are girls, we're singers and we're friends -- that's the main thing about that tour and what's gonna be the most fun," Aly adds. "It's like having a girlfriend with you on tour, really. I think it's a perfect bill."