I'm pretty new here and to keeping snakes as well, but I'm not a newbie to housing things in tanks, as I'm a long time fish keeper. I think the commonalty in both is that you need to make decisions based on what is best for the animal and not what is best for the keeper. So even if it APPEARS like there is lots of room for more snakes/fish in an enclosure, there frequently are a lot of other issues involved in determining what can go together.
Sometimes even if it seems logical or other people do it, it wont work with specific pets. I have angelfish who ought to be able to live together, but can't--I keep them in different tanks for their own benefit. I imagine it is the same with snakes--that some might be able to get along together in some situations, but it is better to do what is in the best interests rather then risk it.
If it is a space/money issue there are lots of suggestions I've seen on here about less expensive ways to house snakes.