Board Thread:Theories and Speculation/@comment-26389094-20150918090852/@comment-25122992-20150918120803

Yeah, Obito had no second Rinnegan. Even Madara had to get his left eye first to cast the IT. I don't know if only Rinnegan eyes were required to awaken the Rinne Sharingan, but I think wielder must have at least two spare eyes of one kind (the main example is Kaguya, who didn't have the Rinnegan at all, but wielding the Byakugan in both of eyes was enough for the Rinne Sharingan awakening). Also I have an assumption that Obito actually didn't know about casting the IT through forehead eye, although there's no facts to confirm this (he had the MS as in Madara's case, so he had to decipher the same piece of information on Uchiha Stone Tablet as Madara did).