sounds like u have really given this a lot of thought and "research." based on this post (and other ones from u), u have a lot of reasons or excuses to get this enclosure and keep the 4 year old Milk Snake and your baby BP together. u came up with all of those reasons yourself. no other forumer has said the snakes won't see each other, the BP won't go to the second story, there's plenty of hides, their husbandry is the same, etc. etc.
again why bother asking? sounds like u already made up your mind.