Board Thread:Theories and Speculation/@comment-35023081-20190918151946/@comment-32685487-20190918212558

To awaken the Rinnegan Madara needed Hashirama's flesh, not just his chakra. He added that flesh to his chest wound. It healed over and eventually he awakened the Rinnegan in both eyes. Sasuke on the brink of death with a hole through his chest and heart was healed by having Hashirama's Cells pumped into his heart by Kabuto therefor Sasuke was going to awaken the Rinnegan anyway. Hagoromo knew this, and when he let Sasuke use half of his chakra he also somehow turned Sasuke's dual Rinnegan into a Tomoe Rinnegan in his left eye alone. I'm sure if Hagoromo would have just let Sasuke use half of his chakra and not interfered with his eyes Sasuke would have had the normal Rinnegan in both eyes during the 4th War.