I definately would not keep a king snake with another snake, because of their behavior. My boas on the other hand don't mind so much. It is rare for Boa Constrictors, specifically Red Tails to cannibalize. I have some good friends who've co-habbed these kind of snakes for 10 & 15+ years with no cannibalism.
This is a good article and they have a lot of sources. In the wild I have found garter snakes mostly of the same size together. I also found an American Black Racer that was about the size of my male corn snake. It was huge and scary because it was mad. It was under a board with two slightly smaller ghost (non albino) gopher snakes. I was scared because I was little and had only ever seen garter snakes, and here were 3 giant snakes that I didn't know about. Seems like snakes of the same size don't mind co-habbing (depending on the species).