Thank you for the clarification, Xue Lan. In that case, I will recommend "Sorcery and Cecilia" by Patricia C. Wrede and Caroline Stevermer. It was written as a series of letters back and forth between the two authors, each pretending to be a Victorian-era sorceress/debutante. There's magic (of course) and a mystery to solve, and the milieu to keep consistent. This is a technique that would be easily adaptable by modern authors collaborating on stories via the Internet.

Each one tells what happened to their character over a two or three day period, then sends the letter off to their friend to tell their next part of the story. In this particular instance, the girls are friends who have been seperated for the summer, one rusticating in the countryside while the other "comes out" in London, so there's no problem (until the end) with stepping on each other's toes or pirating each other's characters.

Most authors would probably do this instead with both characters experiencing the same things (roughly), and each telling from their own perspective how events transpired, but you could do it with the two authors playing the parts of e-mail "pen pals" who live far apart but know some of the same people.