Hi Adam,
Multiple exposures can be fun to work with, particularly when you imply the spirit-like presence of a person within a landscape.My hunch is that you made this photo “in-camera” using the Image Overlay tool in the Nikon D3100.
As an option for next time, you can also create this effect in Photoshop, and if you do you will have significantly more control.
To make this image in Photoshop or Photoshop Elements, go back to the two original images that you used here (the photo with and the photo without your mate), open them both up in Photoshop and then copy (Command-A) and paste (Command-V) one image onto the other.
The simplest way to make one image appear through the other is to go to the Layers panel and adjust the Opacity on the top layer so that you can see the lower layer starting to appear through. It’s that simple.
While you're in Photoshop try experimenting with some of the different layer blend options. Blend options can be found at the top of the Layers panel – just remember to check which layer you have selected before changing the setting.
Keep working on this idea, it's possible to create
some really interesting ideas within multiple exposures.
Cheers,
Anthony