Adding to what rivenwanderer said: it looks a bit like the head of the person in front has been cut off at the top. That may be a limitation of the original picture you're working with, but because it's hair, you might be able to use paint or smudge to restore the curve of her hair on top of her head.

Another trick is to try using some careful blurring at the edge of one image; that can make it look a little less "cut out". It can make it look worse, though, so you have to experiment.