I tried putting it on the outside once and the bearded dragon would constantly attempt to get at the evil twin that resides in the reflection. After taping it to the inside he is now calm.
Once I tried just putting a piece about 4" high on the side where he does the glass dance, but from his basking spot he was able to see the enemy hiding just above the piece of cardboard and made a mad dash for him. Even after reaching the cardboard he was sure that the other bearded dragon was just on the other side and he actually tore down the short piece of cardboard in his attempts to climb over it.