I have had some very food-aggressive milksnakes cohabitate for years with no issue of cannibalism. In all cases, the snakes were raised up together, usually siblings. I even did it with Cal kings, though I think with the getula the chances are greater. In any case, no cannibalism occured. Doesn't mean it can't;just means it isn't the rule.
Introducing a brand new snake in with an adult is more risky IMO. Kind of like what Tad related in his post.
There are good reasons to not cohabitate - keeping cages clean, keeping records, etc. But if you want to do it, cannibalism would be my least concern with annulata. Not impossible, but not the rule.
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Mark