By experienced, it means you can tell if there was something right or wrong with the animals who are housed together. Its the ability to keep them healthy and thriving, knowing when they're not, and how to fix it. Inexperienced keepers just don't have the eye or knowledge for such things yet, which is why its not recommended to house multiple BPs.
Some species of snakes *have* to be housed together permanently in order to breed successfully.
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