If you actually meant a one-way mirror, then yes, I think it is possible, with enough work. When you're on the mirror side, you would have to have a copy of the entire room you're in being rendered on the other side of some glass, within another room. However, this copy of the room would have to a func_illusionary, and you would have to use an env_render to switch it to invisible when you're on the other side of the mirror, looking through at the room with the mirrorred side. I might try it some time, just to prove it can be done.