Just checked on ye olde Google... very, very few digital cameras allow multi-exposures. Take two identical photos with something changed in the photo, but keep the camera position identical. Load one photo and load another photo as a new layer. Play with the blending and opacity options until you get what you want and/or mask out the layer and erase all that you want to remove from the layer and play with your blending/opacity options.
I haven't done it yet, I don't even own PS. Just read it.
