If you have only 1 face-down Defense Position monster you cannot activate the effect of “Shifting Shadows”.
If you control a monster that is targeted by your opponent’s “Nightmare Wheel”, and you flip it face-down with “Book of Moon”, and control at least 1 other face-down Defense Position monster, you can activate the effect of “Shifting Shadows”. However, afterwards you must tell your opponent which monster is targeted by “Nightmare Wheel”.