This is why I never use this argument. It's not a good argument because by removing a snake from it's natural environment, the whole situation is already "unnatural".
That's why I go with a cost/benefit analysis instead. What are the costs and benefits of cohabitation? What are the costs and benefits of housing everyone seperately?
Cohabitation:
Costs: Possible stress. Possible cannibalism. If one regurges, you don't know who it was. If one has funny poo, you don't know who's it is. If one gets sick, there is more of a chance of the other getting sick. Etc...
Benefits: Cheaper to house multiple snakes in the same enclosure. Some people like to see snakes supposedly interacting. (Pretty much the only benefits I can see are those that are beneficial to the human, rather than the snake).
House Separately:
Costs: Have to pay for more housing. Don't get to watch snakes [supposedly] being "bestest friends".
Benefits: No risk of cannibalism or stress due to spacial competition. You know who the regurges and poops belong to. Etc...
Looking at it this way, my decision was quite easy. I just see no reason to keep snakes together, other than reasons that are beneficial to the human rather than the snake. Nobody is going to convince me that their snakes are "bestest friends".