Hi! I am looking for any tips on the mirror in The Who's Tommy?

Some people use scrim for this effect. You can paint or dye it silver/gray. Or, you can make a two-way mirror using plexiglass and window tinting that can be obtained from an automotive store. Home Depot also sells Gila Mirror Pricacy Window Film. They key is being able to control the light levels on the two sides of the "mirror". Bringing up a bright light behind the mirror will reveal what is behind it. Killing the backlight and bringing up the lighting in front of the mirror will hide what is behind it.