So in my sleepy state, I thought up of another theory that could both help and rival the Anti and Schneeplestein theory. What if Schneeplestein was a disguise of Anti? I mean they both have murderous tendencies, and Anti's glitches could be numbed by the power of disguising himself, the scalpel is the knife of the medical world and Anti's primary weapon we see is a knife. You may ignore this as I was probably just loopy from being so tired, but it's slightly plausible.
Though I see them as 2 separate egos/ characters, it would certainly be a clever disguise for Anti to use temporarily. You are correct on their related “murderous tendencies”, and those with similar traits often form… Alliances.