Co-writing and setting deadlines are great for inspiration. We're constantly tossing "what-ifs" back and forth. Knowing that we have deadlines keeps us accountable.
We save to Dropbox and write at different times of the day and night depending on our schedule. Either one of us may open the current work-in-progress to find new scenes and dialogue we didn't write, which leads to our in-joke about magic books: "Look! I went to sleep and the book wrote a chapter itself! Sweet!"