The simple answer to this question we're often asked is that we write in the first person, with each of us taking responsibility for our own characters in the book. The way our process works is that together, we come up with a basic story outline and a cast of core characters. We flesh out the characters and decide which ones we'll each 'own'. Then we write the basic story down in dot point format, with each dot point representing a chapter and character. Having done this, we then start writing. One of us writes the first chapter, then sends it to the other, who then writes the second chapter, and so on. It's great fun, because when each of us receives a chapter from the other, there's often some surprises in there as the story takes shape and new twists and turns get added to our initial storyline.