Recently, I shared some of my double exposures created in Photoshop. It's really easy to do. The tough part is finding just the right photos.
1. Open two photos in Photoshop. I like mixing portraits with nature shots or landscapes. It's best to have one photo with lots of white/light colored negative space and the other to be darker with the entire frame filled.
2. Drag or Copy + Paste one of the photos onto the other. You should now have two layers. On the top layer, change the Blending Mode to either Screen or Soft Light.
3. That's it! You can always change the opacity or make other adjustments. For this photo, the Screen blending mode was used. The photo on the right at the very top used Soft Light.