As presented here, the two-way mirror would be ineffective with Frasier and Roz in a room with the lights on. Two-way mirrors only work when the viewer's room is dark. In other words, as shown, each side would be equally visible to the other.
As Frasier and Niles walk from the kitchen musing about a focus group expressing an opinion about Freud, the camera takes such a wide shot that you can actually see the stage lights above the set.
Niles: Frasier, not to appear unsympathetic but, I've had my 'I'm dangerous face' on so long it's starting to hurt.
Frasier: (describing his contempt for the idea of a focus group for his show to Niles) I mean can you imagine Sigmund Freud being dragged into a room full of Viennese laymen to hear remarks like "Hate that Oedipal thing but love the penis envy."
Manu: All this because you had to know why I do not like you.
Frasier: Well, you were kind of vague.
Manu: You want to know why? I'll tell you why. I think you are a smarty-pants. I was too polite to say that before. But then you spied on me, you stalked me, you badgered me, you poured Yoo-Hoo down my Dockers, you crunched my hand, and then, as if all that was not enough, you burned down my newsstand, my livelihood, a gift to me from my beloved uncle, may he rest in peace!
Frasier: Was that so hard?
Martin: Hey, Frasier, how was the focus group?
Frasier: Fine, I guess.
Martin: Oh, well, you said yourself they didn't mean anything.
Frasier: Yes, well, it's one thing to say that. It's another to sit there being personally attacked! One man actually said, "I just don't like him."
Niles: Only one?