User blog comment:Omega natsu2/Chapter 370: Demons Reincarnated, Predictions./@comment-24467372-20140205201110/@comment-7418318-20140206090914

Right. Madara did say he was still missing his second eye, which is either in Obito's skull or stored in that other dimension, which is only accessible through the use of Obito's and Kakashi's mangekyou sharingan. Either way he needs Obito to get it, or Kakashi for the latter case, but they are both at the same location as far as we know.