While that petshop is probably doing it all wrong and for the wrong reasons, on multiple occasions I've found more than one corn snake peacefully occupying the same retreat. For the most part, cohabing is a balance of risk and reward that any interested keeper must assess. Just about any snake can either kill or be killed by any other snake if they're thrown together. Most, except bonafide ophiophagous species, don't just slither around killing other serpents.