The “double exposure” technique was already used in traditional film cameras. There is a cool short video available on Youtube, go to https://www.youtube.com/watch?v=DIKzso0_WYo .
Some of the latest digital cameras can do double exposure otherwise you can do double exposures in Photoshop or Gimp as well. There are even apps available to do double exposures on your cell phone. I kind of like to photograph double exposures in my camera and sometimes get to be surprised by the results.
This is a simple double exposure created in my digital camera. I photographed a sleeping gentleman in the park as my first image and then the main street in Mariposa, Ca as my second image. The camera combined the two images into one.
Enjoy and make some great images!