Gee, why not just give him the definition while you're here?
Quarantined, as in kept isolated from your current snake to avoid any possible transmission of parasites/diseases. Just give the new acquisition 2-3 months so you can monitor behaviour, fecals, and sheds before intoducing them.
While I'm sure Cheryl's snakes are 100% healthy, there's really no way to tell for sure in the grand scheme of things. It's just the safe route, most of us take.
As for snakes housed together at petstores...petstores don't always practice the best of animal husbandry. Most of those corns are probably curled up together in the only optimum spot in the cage. That is...one hide, one lamp for warmth, etc. They're all after the same ideal.
As DaemoNox said, most corns ignore each other unless its mating season. So it really wouldn't be all that exciting of an encounter. Most of mine when together for picture shoots do their best to get away from one another as fast as possible.