sometimes treatment providers start more than one medication at the same time. its called a "medication cocktail" (to wrap your head around this idea think about how some people to get a desired affect drink alcoholic cocktails which is a mixture of different alcohols in one drink) not all anti psychotics work for a whole mixture of symptoms. so treatment providers use a book called the PDR and other manuals that contain information on all the prescription medications. using these manuals treatment providers know which meds work best ...in general.... for which symptoms and dosages. they mix and match to get the desired combination that is best for the patient's problems then adjust the cocktail depending upon the feedback from the patient\client based on why they prescribed each of the medications in the "cocktail" both of those meds may be anti psychotics but they each have their own properties that work best in the way of which individualized symptoms is happening.