Separate it like so:
A func wall with tranparency or transparent texture that encases an solid object with SKY texture on the side you want to see sky and BLACK texture (thats the name of the texture) on the side you dont want to be seen, and add another solid object behind it (on the side you want to block visibility from) and it will make it like so - The ones seeing sky will only see sky, the other side will only see black.
An example of this is used in Half Life single player in the map after the hangar with a whole lot of lasermines that you escape via an elivator. Look at the windows in the armory and outside of it and you will see what I mean. Other than that, separe the two rooms with teleports if its not nececary to be connected physically in one swop.