u don't mean you're gonna house your BP and the Milk Snake in the same space right? u mean your BP in one story of the enclosure, the Milk Snake on another story and the third one being empty? it's not a good idea to house a BP with another snake regardless of species, size or sex just b/c of their personality and how easy they can become stressed.
i made a thread asking about Corns vs. Kings/Milks before i got mine. I asked if it would be a problem for my BP's. i have two baby Milks now and they are in the same room with some BP's no problem. but i don't handle them together or even handle/feed the Milks after handling my BP's or Corn. and they are in separate enclosures of course.