What is the side effects of benedryl and Vistaril when taken together?

I dont know the doses, Im not sure if anyone does. But I'd like to know what may happen when taken together. Im pretty for sure that the two should never be mixed, but my mother in law swears that it wont hurt, that taking benedryl along with Vistaril helps the Vistaril work as a sedation medication.

I think she's wrong.. but what do I know.