When in doubt, consult 2ch
They call the unknown song "Watashi ga obasan ni nattemo" and apparently it was Manami & Nana's duet.
& Damn these girls can sing, especially Manami ;_; If they ever ever debut officially, they're gonna pwn so hard. And their MCs are hilarious! Not that I understand, but the audience seems amused and Nana sounds so clumsy
lol.