Frontline spray (not the concentrated drops; this is important) as sold for dogs and cats. Take the snakes out, lightly mist everything in the enclosure and allow to dry, then replace snakes and dab a small amount of Frontline spray on the last third of the snake's body. A herp-keeper at a zoo told me this trick, and I used it when a new kid turned up with the creepy crawlies, and it worked great. One application seems to have completely eliminated them, although in my case it was a brand new snake and so had not had time to develop a big population of mites in the environment. In your case I would re-treat in a month or so just to be safe.

There is also a product called Provent-A-Mite, referred to here a lot as PAM which many people are very happy with. My beef with it is that it is pyrethrin-based and I really don't like using those chemicals on any of my kids, and it is only available online while Frontline spray can be purchased at any decent pet store or from your vet.

Also...it is generally considered safer to keep one snake per enclosure. They tend to do better, and you have better control over any problems that may arise (regurgitation, upper respiratory infections, parasites, etc).