Well, yeah, it wouldn't make sense for a lot of items, but in the books, at least, the rings are explicitly noted as having several charges (or, put another way, he's got a dozen small items that all do the same thing, once), and he uses several at once to distinct effect.
I guess in most games, it'd be more likely to have a case of using two distinct items at once (like, say, going guns-akimbo with prepared wands). As a note, it's not really my idea; as I said before, I'm just trying to figure out how something from the books would be modeled in-game.
I totally agree that it wouldn't be a good idea to let it get out of hand--if there's any way it would be implemented, it certainly wouldn't be strictly additive.
And d'oh, yeah, screwed up the math on that one.