Someone else on the internet pointed out that Cobb's tradition of violating his own rules extends to his totem as well, in that he never wears a wedding ring in the real world, but has one present on himself in the shared dream spaces, so his totem is not in fact an actual real physical object, but the absence of one. He uses Mal's totem primarily to check to make sure he isn't in his own dream, because if he was he'd be able to will the top to spin forever, where as in reality it would eventually fall over no matter how much he willed it to spin. He has this extra totem because he regularly uses the dream share device by himself.