From the current chapters of Naruto, it is revealed, that Rikudou Sennin has a brother who also sealed the juubi in himself. That means there are two juubis in the world. If Madara has/had control of one of them, where could the other be hidden? His presence in the world for now is speculation. But as the Rabbit Goddess had Byakugan, this could mean that this brother may have been the Hyuuga's ancestor, which would mean Senju, Uchiha, and Hyuuga are indeed related, but Hyuuga and Uchiha could never be able to use the other's doujutsu, because of difference in ancestor.
So in essence, I'm asking, where could the second Juubi presumably be, is it not possible that the brother is the Hyuuga ancestor? For now this is all speculation, but I have a feeling that Kishimoto will end this series with a few plot holes, so I want to see other users' ideas on these questions.