How do you blend two faces together?

There is also a technique using auto blend, by masking out the areas of each image you want to exclude, but leaving a good amount of overlap. There are tutorials, on this, on Youtube.

Its not just a one or two minute explanation. Maybe look for some tutorials on YouTube? The basic idea is to line up and size the images then use masks to fade from one to the other. Your reference image was done by a professional retoucher or artist and probably used photos with matching lighting that were created specifically for this concept.