I might as well ask a question. Do you think an uchiha would need more than one eye to use Izanagi?
Obito knew about it, so Madara must have taught him, but Obitos eye was also special, and Madara must have known. I can only think Madara taught him how to use Izanagi in case he made a mistake, so when he went up against Minato and Konoha he had another eye, probably Madaras own eye, but that would need him to learn how to implant it as well, since I don't think it can be done while your blind.. Can't be that easy, maybe for Madara it's an exception, but still. We also saw Obito implant later so he must have learned. And of course we saw he had another eye hidden under the mask against Konan, but what would his reason to hide his other eye be, if it was one without special abilities?