This process just takes two photos, but you should choose carefully which one you want on top and will make transparent. Remembering to copy your background image, copy and paste your top photo over the background to create a second layer. Then drag the transparency slider until you have the "combined" look you like. If the top layer doesn't quite line up to be where you want it, adjust it to where it fits properly and then crop the two layers together and save the image as one image. You can also play with the settings of each layer to darken/lighten, adjust gamma, or even add a filter. I like doing this best in grayscale, but some interesting color effects can be had as well.Try it! Things like faces over landscapes or figures floating on the water make interesting "multiple exposures"! Here's my quick sample.
