Since this question keeps popping up, I decided to address it in the form of an interview. I took the most commonly asked questions and answered them!
Can readers tell which of you wrote which parts? No. Since both of us edit each other's sections, the writing styles blend. For fun, we've had friends try to guess which of us wrote which parts – they never are sure!
Have you tried co-writing? Are you about to? Do you like or dislike co-written novels? I'd love to hear your thoughts!