You can always use the Rayswitcher material to fake that effect to the extent you want it artistically.
Doing it like in reallife:
In reality a one way mirror is a semi transparent and reflective surface, where one side of the room is dark and the other bright.
You can look from the dark room to the bright, since a part of the light passes through the glass and there is nothing to reflect in the dark room.
Vice versa the bright room has lot's of light to reflect but the dark room has none to pass through.
So making a window 90% reflective is already enough if you want it to be like in the real world.
A normal window is almost a one way mirror already. During the day you can easily look outside you room, but it's hard to look inside a window during the day, since the outside reflects so much.
Vice versa during the evening you can look inside every house, but looking outside from the house is dificult, since all the lights inside reflect on the window so much. If you would raise the reflectivity (by incresing the Fresnel IOR) any window would become a one way mirror.
