Just use a one way mirror. Get some thin mylar and some clear acrylic. You could experiment with adhesives to find one that won't fog the mirror or just wet suction stick it. As far as I know teleprompter mirrors are just glass at a 45 degree angle in front of the lens with a backwards printing TV (or twice reflected TV) perpendicular under the lens.
The part of the white lettering is reflected and visible against the darker background an dpart of it passes through skyward. The camera, giving off no light, does not cast a reflection.
Another option would be to film it in two different shots: one in the interior of the interogation room with a real mirror where the 2-way would be, then for the scond shot, have your "hot set" be the viewing room and have the actors replaying the events of the interogation room behind simple glass with darker lighting in the viewing room or maybe a slightly tinted glass. Be careful of your lighting using this suggestion though, too much light passing into the viewing room from the interogation room would give it obviously fake feel.
Also, remember that most interrogation rooms are small, and consist of nothing but a chair and maybe a table. Make the lighting real harsh, and make the room real drab, unless you're going for a freaky effect, like in The Matrix.
