Yeah, that was a pretty clever optical on Argento's part. Of course when you watch it now it's awfully quaint and dated but it worked well at the time.
This may not be Argento's best flick but it's definitely one of his craziest. The last twenty minutes especially -- the world's longest telephone cord, the pool of maggot-ridden corpses, the guy breaking his thumb to get out of the cuffs, Daria Nicolodi doing a sheet metal decap, the monkey with the razor, the deformed monster kid eaten alive by flies...pure craziness, I loved it!