Sara K Byrne is a wedding photographer who has come up with a fantastic video tutorial on creative use of the new multiple exposure feature on the Canon 5D MKIII.
Double exposure is a technique that originated with film photography where you would expose the same frame of film twice (or more). Film can only be exposed to light so much before it will stop recording information. So the part of the film that was darker after first exposure will be most receptive to the light from the second click. It’s typically good to underexpose both photos, because you are exposing the “film” or “sensor” to light twice.
Be sure to check Sara’s blog and website at http://www.sarakbyrne.com for more information on this and other techniques and to check out her fantastic wedding captures.