That's certainly quite a plausible answer. I remain unconvinced that there's much point in distinguishing between A's action and B's, since for both of them (1) the agent intended to kill C, and (2) C did in fact die, and (3) C would still have died without the other person's action. But if for legal purposes it were necessary to set down one of them as the killer, I expect your answer is as good as any.

(What if B, instead of putting a hole in C's water bottle, put in a substance that is poisonous on its own, and that reacts with A's poison to make yet a third poison? If you still think the answer is that B was the real killer, is your intuition disturbed at all by considering a scenario where instead B puts something into C's water bottle that's unpleasant but not deadly on its own, but that reacts with A's poison to make a different deadly poison?)