Ok, well you could either take a photo with two people standing behind each other… :P
OR
take two photos, one with each person, then take the second photo, erase everything but the person so everything
is transparent, then copy it into the other photo.
After that you just use the erase tool to remove any part of the second photo that is on top of the first one, then
TA DA you have your second guy sprouting from your first guy
Timmy's method will work or you could get the two images and place a mask over the top one and paint out the areas you don't want on the mask. This will be non destructive and allow you to make easy corrections as you go along.
The more I use photoshop the more I realise there seems to be a million ways to do the same thing The best is just personal preference in most cases ... (I do love a layer mask tho)