You could just avoid the whole problem by braining the rat in the head before feeding it to your snake, or skip live alltogether and switch to frozen thawed, much safer all around, and you can store the rats without having to feed them.

But putting the snake into the rats cage sounds like the snake would be less likely to feed due to just being handled for a long period of time before being introduced into the cage...

Plus why leave the rat in the snakes cage at all?