Let me ask you this: what are your favorite co-authored novels? I'm especially interested in those with an epistolary format, or with a format where there are multiple points of view or storylines and the two authors are crafting those different points of view. I'm going through all the ones I can think of, love 'em or hate 'em, looking for good examples of how these are structured and what kinds of stories they tell.
Some examples that clearly fit this and come to mind right away:
Sorcery and Cecelia, by Patricia C. Wrede and Caroline Stevermer. (Charming!)
Dash and Lily's Book of Dares, by Rachel Cohn and David Levithan. (Schmoopy!)
Nick and Norah's Infinite Playlist, by Rachel Cohn and David Levithan. (Romantic!)
Which Brings Me To You, by Steve Almond and Julianna Baggott. (Off-putting!)
Some other co-written novels with this kind of episodic format that have less clear-cut roles for the authors:
The Guernsey Literary and Potato Peel Pie Society, Mary Ann Schaffer and Annie Barrows. (Delightful!)
Illuminae, Amie Kaufman and Jay Kristoff. (Thrilling!)
I can think of others that might fit that I haven't read--David Levithan co-wrote a couple of other novels that I think had similar divided approaches (You Know Me Well with Nina LaCoeur; Will Grayson, Will Grayson with John Green).
What else can you think of? What do you have for novels (especially brisk, well-plotted stories) where two authors tell a story in partnership or in parallel through two characters?