Good point Don.
Let me see if I have this straight. The reasoning behind the rule is if someone were to lift a neutral in a panelboard that was doubled up with another there would be a hazard of causing sever voltage imbalance at down stream loads, right? 408.21 only applies to switchboards and panelboards, Right? 408 does not cover disconnecting means making this an acceptable installation. (The lug is duel rated.) Lets say, hypothetically, that the load end of this feeder lands in a panelboard where it continues on to another panelboard via either a breaker or a bus tap. Now lets say the where the grounded conductors come together in this first panel from the disconnect and the sub panel, they are landed under the same lug. Would be a violation of 408.21 or not? How would this be any different electrically than the situation shown here? For that matter, what if this were a panel instead of a disconnect. It would then be a violation? Or is it not because it is a splice. Where do you draw the line in calling it a splice and two separate conductors?