You're to be commended for checking first on behalf of your sister's snakes! I wish everyone cared so much. As other have said, it's possible to split a big a tank for two snakes. I currently have a 40-gal split, in which I'm housing 2 yearlings. They're both female, just in case one manages to get to the other side. I betting that's a "when" rather than an "if."
Don't, DON'T, DDOONN'TT house a king near a corn. I'd put them in separate rooms if possible. In nature, they're predator and prey. You probably wouldn't set up housing for a parakeet and a cat, right? Even if the parakeet was out of reach, having a predator right beside him all the time would be a definite source of stress for the prey animal. It's bound to be the same for a corn snake, although they don't exhibit stress in the same manner as a warm-blooded animal.
Good luck with the snakes. And good on you for asking! :cheers: