Unfortunately one way mirrors do not exist. The principle of a "one way" mirror is that you have a piece of glass that is partially reflective. If you then look from a dimly lit room through this glass to a very brightly lit room then the light from the bright room will pass through the glass and wash out your own reflection. On the other side of the glass, the reflected light from the bright room washes out the dim light coming through from the other side, giving the impression of a mirror.
So if you were to apply this to a puzzle then you would just need to use transparent stickers.
oh ok, that makes sense. It's to bad "one way" mirrors only work in dark conditions. It would've been really neat.