Thursday is Q&A day!
Duplicate your original image of your first animal, grab the Clone Stamp, choose soft tip brush and press Opt/Alt-click to define the area to sample. Now start painting over the head. Place the second animal’s head over the first, and mask.
Now, grab the Smudge too, set the Strength to 60 and choose a small-sized brush. Try quick and short moves to start pulling the fur around the second animal’s face to blend the two together at the neck. Finish off with custom brushes to create whiskers.