Kamen Rider x Kamen Rider OOO & W feat. Skull Theme sang by Koji Kikkawa & Maki Ohguro ~ CSToys Blog, We're With Heroes

A great news for everyone. Koji Kikkawa who played Narumi Soukuchi a.k.a Kamen Rider Skull in the upcoming movie Kamen Rider x Kamen Rider OOO & W feat. Skull Movie War Core, will be singing the theme song of the movie together with OOO's opening theme singer, Maki Ohguro. For now, we only know that the song will be released in December 15. As for the title, it is still unknown yet.
So, do watch out for more updates on this! ^_^
Source: Jefusion | Ukiyaseed | Alternate Zero via HJU
Original post at my blog: GideonBurogu

There is actually an article about this. However, it's only in Japanese. But rest assured, as I will give you a translated summary.

Kamen Rider OOO & Double, the theme song is 'Koji Kikkawa × Maki Ohguro'

Is this the ultimate rock collaboration!? It's been decided that Koji Kikkawa and Maki Ohguro will together make the theme song. Together they've formed a temporary unit. The lyrics will be written by Maki, while Koji composes the song. Together they'll be the Double vocals for this movie that premieres the 18th December. This will be the ultimate collaboration, with Koji that also plays Kamen Rider Skull and Maki who made the opening for Kamen Rider OOO.

Maki said that before she made her debut, she had wanted to become an artist like Koji Kikkawa. (Maki debuted 1989, while Koji Kikkawa debuted 1984 with the song Monica.) She also said "I am literally dancing from joy to be able to sing with Koji Kikkawa."

Koji Kikkawa said that he's liked Maki's songs before this was decided and feels like he's able to study a womans heart through this collaboration. They both seem to have a mutual trust and respect for each other.

Source: http://animation.blog.ocn.ne.jp/anime/2010/10/post_1045.html

Edit and translation by DecaEnd.