So lately in Torts we've been reading about Causation and the "But for" test. (That is, "but for" or "without" the defendant's negligence conduct, would the "accident/injury" to plaintiff still have occurred? If so, then the negligent conduct was NOT the cause of the injury and hence no liability. If not, then the negligent conduct was the cause of injury).
However, how does "but for" apply when two separate negligent acts are at play. For example, Defendant A's polluted water leaks into plaintiff's pond and kills all his fish. A few hours later, Defendant's B exact same polluted water reaches the pond but the fish are already dead.
Under the traditional "but for" test, this would seem to negate Defendant A's liability.("But for" or "Without" Defendant A's negligent act of allowing his polluted water to reach plaintiff's pond, Defendant B's polluted water would have killed all the fish.). But how can that be so, since Defendant A was, in fact, the cause of injury?