No need to be rude, guys...shoot, I had the same question when I started out with snakes. =)
The rule of "one snake + one cage" is good husbandry for all species of snakes. Snakes in cohabitation (except for breeding) can lead to numerous problems:
1. Stress. Two or more snakes housed together will be after the best "hunting" spots, the best hiding spots, the best basking spots, etc. Again and again as seen with snakes kept together, one snake will usually be dominate over the other, which can manifest the other snake to go off feed, to be less handle-able and ultimately, can lead to cannibilism. To get rid of the stress of the other snake, one may very likely eat the other, even if they are the same species and size, even if the smaller one eats the bigger one. And many with cannibilism, especially with the smaller eating the larger, it leads to death of the eater (as well as the eatee, obviously).
"But they look happy because they cuddle all the time." This isn't cuddling; it's one snake showing dominance over the other. For example, a good friend of mine purchased an adult pair of Vietnamese Blue Beauties which had been housed together their entire lives. The female, while sweet as can be to her owner, was totally dominant over the male, always making sure some part of her body was on top of the male's head. It looked cute, but my friend has reported that the male, now in his own cage, is finally putting on weight and is obviously getting healthier and happier being on his own.
2. Husbandry problems. In this example, cornsnakes and ball pythons, it will be impossible to provide the correct husbandry, that is humidity and temperature gradients and ranges, required by those species in a single cage. Ball pythons are orginally from tropical western Africa, whereas corns come from a range of the US that is generally much milder. A cage set up for the husbandry requirements of one snake will most likely lead to health issues and death for the other.
For more information on husbandry, please check these sources for good caresheets:
Link to cornsnakes.com cornsnake caresheet
Link to ball-pythons.net ball python caresheet
3. Transmission. Unless you are getting the snakes from absolutely trustworthy breeders, and even then, it is good practice to quarantine all snakes to verify that each animal does not carry any parasites or health problems that can be transmitted. The big one here is mites. I personally experienced mites (may they rot in hell) and had I not quarantined my new animal (from a reputable reptile store) that turned out to have them, the mites may very well have spread to the rest of my snake colony.
Other transmittable cooties can come from wild caught snakes, especially within both the ball python and cornsnake species. It's easier to avoid with separate cages and quarantine procedures.
4. Pregnancy. You may be pretty certain you know the gender of your animals, but until they lock up and one lays eggs, there are still slim chances that the probing or popping was incorrect (although this may be more prevalent among ball pythons, from what I've read, or just people who were bad at sexing, hehe). Anywho, two snakes together can very well lead to breeding, which can lead to a gravid snake and babies. It can happen, as odd the chances would be for a ball and corn to breed, why even make it a remote chance? =)
So, these are the reasons why snake cohabitation is a no-no and why people get so hot under-the-collar about it. You are, of course, welcome to make your own choice, but fellow snake lovers and owners hope that these arguments explain why it's better for "one snake + one cage". =)
