There is no need to feed your BPs in a separate box. In fact, it can do more harm than good. Unless, you are keeping these snakes in the same enclosure. You aren't housing them together, are you?

I'm sure they could both handle adult mice in the 20 ish gram range. Most juvenile BP's graduate to adult mice or rat pups pretty quickly.

Your best bet is to purchase a scale. It really comes in handy for measuring the weight gain progress of your snakes, and for weighing prey items.

I would not try to de-worm any mice you hold over. It could cause more harm than good, and is really not needed to make them more nutritious. They are completely nutritious just as they are.