Just like in the real world, everything is indeed double sided. You might be able to use the FrontBack map to make something completely transparent when viewed from one side by feeding it into the Opacity (though this will depend on how the normals are set up, as to whether it works - EDIT that is, if some of the objects have one side counted as the Front, and others have the other side counted as the Front, then you would need to set up more than one material for that, is what I mean there END EDIT). You could also use Corona Distance to make an object that clips or cuts away some objects, using a "guide object" to determine which bits get cut away (but that won't be automatic from the camera, it will need the guide object that determines the clipping / cutting effect).
