In my opinion, which I'm basing on things I've read and things that I know large breeders do, I feel that snakes should never be housed together. Here are some points that I keep in mind and reasons why I do not house snakes together.
1. The presence of another animal in their space represents competition, competition leads to stress.
2. Stress leads to loss of appetite and or disease.
3. If one sick gets sick they can both get sick.
4. How do you know who is defecating and who is not?
5. Snakes are not social animals so although captivity its self is not natural forcing two animals to live together is even more unnatural.
Now keeping two different species together can open up a whole new can of worms. Different species can be more susceptible to different diseases. Also they can differ in heat and humidity requirements.