Eulenspiegel, there is nothing wrong with feeding 2 mice at once. Nothing says you have to or even should change to rats when the BP gets too big for a single mouse.

As far as the OP's question, Kaorte's sandwich analogy is great, but I don't think it goes far enough as to explaining why. This gets into some scientific stuff that I'm not sure on the details, but I believe this is at least approximately correct.

Snake's bodies are designed to be able to shut down their digestive system when it is not in use. This helps them conserve energy and is part of how they can survive on so little food. They are also capable of eating incredibly large meals, but this is a real strain on their system. In captivity, we have the luxury of feeding our snakes meals that are big enough and spaced far enough apart to allow the snake to finish digesting and shut down its digestive system before the next meal, but small enough and close enough together that it can continue to grow and never have the strain of a huge meal. So the object is to find the perfect balance in the middle. I don't believe anyone knows exactly where that balance is, but over the years, BP keepers have repeatedly observed that their snakes do well with meals that are about the same size as the widest part of their girth, and spaced about 7 days apart. Young BPs seem to be programmed to grow very quickly at first, probably to get big enough to be less tempting to predators, so feeding them as often as every 5 days works well.