Double exposures are a really fun way of collaborating on a (long-distance) photo project. All you need is a project partner, an analog camera and some film. You shoot the roll, rewind it and then send if off to your project mate. They shoot it again, have it developed and share the results with you on Flickr.
Whether you already have someone in mind or are still looking for someone in the Flickrverse who wants to give it a go, head over to one of the film swap groups. They are the perfect place to share your experiences, find a project partner, or showcase the results of your collaboration.