Board Thread:Naruto Discussions/@comment-34436445-20180703141210/@comment-36351381-20180729052058

It's probably because his body and mind (without possessing Asura's chakra, or the other half of Hagoromo's chakra) can't handle two rinnegan, as we see clearly in the case of Obito who isn't, or wasn't even, either one of the reincarnates. Nagato accomplished this fete (possessing two Rinnegan) as a result of him being a descendant of the other half of Hagaromo's chakra (his Uzumaki-Senju lineage). Their physical vitality is more than capable of handling two Rinnegan. So in short, (in the case of an Uchiha, who is mind only), to possess both Rinnegan you must first have both body and mind.