CREATING A 2-WAY MIRROR
Idea #1 - The mirror needs to be half-silvered. I think that Rosco do such a material.
After that, it's down to lighting. When it's black behind the mirror, it will look and work like one. When whatever is behind the mirror is lit, it will appear. It's rather like doing gauze effects.
Idea #2 - Quite easy, and cheap, to achieve. I used a sheet of Perspex and covered it with mirror film obtainable from car accessory shops that is used on car windows by `boy racers`. Very effective. Reflective from the front as a mirror and becomes transparent when lit from behind.
Idea #3 - Rosco Shrink Mirror Scrim - apply it as tight as you can, then stretch with heat from a hot air blower of some sort to pull dead taut.