I think you're pretty safe with BPs out together, as they are very chill snakes, while many others have stronger predatory behaviors. Always remember
their keen sense of smell...I believe snakes can smell the tiniest bit of rodent scent from a recent meal on another snake's face...and I've seen many
snakes appear to wipe their faces after eating for what I'm guessing is precisely that reason (so another snake they might encounter doesn't make a big
mistake and bite them)? It also makes a big difference if you have a pair of hands ready for each snake that is out.
As AbsoluteApril said, king snakes cannot be trusted, not even with their own kind, & not even with mating.