You should mist them twice a day, it will make the humidity rise which will help with the shedding, just don't soak the enclosure, just make it moist. As well, if the snake is having that hard of a time shedding, you could put them in your bath tub in warm water and let them soak for an hour or two. The skin should then be loose enough to peel off. Also, one thing that may really help is "eco earth" it is an expandable dirt that holds humidity well, and they can also burrow in it like they love to.

As for housing them in the same enclosure, it is okay if they are about the same size and kept very well fed. In that case, it would be okay to give them that extra rat a week.