I'm not joking.
I mean, clearly the sensible behaviour would be that Watchtower worked in the intuitive way when you gain Cache, letting you, for example, topdeck the Cache and trash the two Coppers. But I'm not quite sure that's a consistent interpretation, unless there is indeed an explicit ruling that when you gain cards you handle the on-gain effects of each one (in an order of your choosing) before gaining the next, even when told to "gain 2 Coppers" as a single instruction.
However, you've previously given the example that if you gain Border Village, then choose to gain a Duchy before reacting with Watchtower, Watchtower then loses track of the Border Village and can no longer do anything about that gain. So by my understanding if you did gain both Coppers before applying on-gain triggers for either, Watchtower would, lose track of the underneath one?