TH should work, unless the item is dropping on a fixed ratio with another item, where you always get one, but never none or both.
Nah, those aren't the only cases where it won't work

Is this sort of the same principle that applies to when you get a yellow proc, on the mobs they added single +2 drops too later in the abyssea life cycle, not effecting said single drops?
For some of those mobs, grellow is required to "unlock" a certain +2 drop. For instance, Carabosse can freely drop a Voyage Card but will *never* drop a Voyage Coin unless you grellow it.

Ovni on the other hand has no such restriction, it can freely drop both Voyage Stone and Voyage Card without grellow.