With the thought of outlining, having the two stories outlined side by side so you know when each element will be presented, to be sure when you tell Matt's story you know what you've already told the reader of Katherine's and what you might need to expand on because you haven't gotten there yet, and vice versa. Plus, if you do intend to alternate chapters, having the side by side outline can ensure you have the two stories meld together neatly, rather than have two separate stories that have some connection and happen to be told in the same book.