There's a sense of urgency when you have someone who's depending on your words in a timely fashion. I have a couple of projects I'm doing this way. It's no guarantee that you'll get the work in, but it can be a huge incentive.
I'm about to dive into a story universe I'm working on with a different person, which will help me get some words down this evening. And I've already got my minimum words for the day in with yet another partner in crime.
Have you ever co-written anything? How did you do it?