two way miror Tweet

[CANDITO]

Hello,

in one of the project i'm working on, my client wants me
to do a box transparent in the outside and miror reflective in the Inside
(like a two way miror, but with no single sided polygons, it looks
impossible to not see the miror Inside the transparent sides...
Anyone has an idea how this could be achieved?
For today i will deal with two renders and a mix on photoshop, but
it would be great to have this done directly in the render.
Thanks a lot in advance

Oliv