One of my specialities when working for Olan Mills was double exposure portraits. While the level of creativity by this guy far outshines my work, it isn't all that difficult. My favorite portrait is of my (now ex)wife sitting on her own shoulder. I shot it with a Yashica D 6x6 TLR. It doesn't require much more than a plan, and a china marker to draw on the ground glass to position the exposures correctly, then doing a normal multiple exposure.