Correct me if I'm wrong, but I believe the NEC specifies that a sub-panel has to have separate neutral & ground. Do I take it that a sub-panel in a separate building is treated as special case and is exempt from this requirement?

yes, that is correct...

In this case (#2), I can see that a parallel ground rod would certainly be desirable.

Yes, i agree, think of it as a new service.

In case #1, is the second rod an NEC requirement, or just common practice?

The language reads hard here, but most agree that G-rods are required.

G-rods, and other grounding methods supposively exist to , as the NEC puts it, address 'voltage gradient' and 'lightning protection'. We, as field electricians in this forum could quite likley post many conflicting scenarios to this end....

electure's short version says it best [Linked Image]