Put each face on its own layer. For sake of example, we’ll say the human is the top layer, the cyber-zombie is underneath. If you just want a fast, easy transition, add a layer mask to your human layer and do a black-to-white gradient in the layer mask. Where it’s black, you’ll see through to the cyber-zombie, where it’s white, you’ll see the human. Be a nice, smooth (but very straight) blend between the two.
If you want it to look a little more natural, you can use the Pen tool to draw the dividing line between the two, following the curves of the forehead, nose, chin, etc. Create a selection from the path, feather it heavily, and use that selection to make your layer mask. You can blur the mask further if the edge isn’t smooth enough for your liking. You can also go in with a soft-edged brush to adjust the mask in certain areas if you’d like.
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It is easier to destroy than to create.
More fun, too.