I have to merge two faces into one. I have both the pictures on Photoshop, but I don't know how to put one behind the other. Once I do, I can play with the opacity to make it work. HELP PLEASE! Thanks.
Here are a couple of suggestions to add to Jane-e's post above. Once you put both images into the same file on different layers, you will need to resize the larger face down to the size of the smaller one (better to shrink the larger one so that you don't get image degradation by adding pixels.
Then you can add a Layer mask to hide everything—with the top layer selected, hold down the Option key and click the Layer Mask icon at the bottom of the Layers panel.
Next, use a slightly soft edge brush with White as your foreground color and start panting in the details of the face you want to add. See the example below: