I think that this is something that CAN be done but for the most part, shouldnt be done. I don't think that there is a problem with keeping a few of the same species together (particularly with non territorial species like nearly all snakes) as long as there is enough room, hides, basking areas, and separation during feeding. If you are going to mix species then all of the above should be applied and you should also try to get species that inhabit different environmental niches. For instance, a heavy bodied ground dwelling snake and a slender arboreal snake that are from very similar habitats would probably do alright together given enough space.
I think that the argument of multiple snakes is taken to the extreme. Yes, some snakes will not get along but if they are found together in the wild commonly and if they do not eat each other then the amount of stress is minimal (and when you think about the huge difference in the amount of stress such as predators, drought, inclimate weather, etc. a wild snake goes through compared to a captive snake then it is really hardly a factor).
Bottom line is that you have to know the animals very well before you can really make an accurate assessment of whether they would do alright together.