The ball would have had to have been signed by Drysdale before his death in July 1993. If he had signed the baseball before Koufax, it is likely that he would have signed on the sweetspot, so in all likelihood Koufax would have already signed the ball by then. Below is a sample of Sandy's signature from 1993 for comparison (Koufax's signature has changed dramatically over the years). To my eye, the slant on the "Koufax" is all wrong on the baseball you are evaluating.
Rick
koufax signature 1993 -latest style for Drysdale dual signature.jpg