It recently occurred to me that Tobi, whether or not he was at one point Madara Uchiha, has to have been melding his body into other people. Remember the Tobi we saw controlling the Mizukage was identical to the Tobi who helped Itachi kill the Uchiha clan and showed Kisame the same face which he later identified as Madara, but Madara was long dead before the Uchiha clan massacre.
This is further complicated by the fact that the Tobi who fought Minato was using Obito's Kamui.The fact of the matter is that the Tobi who met Kisame couldn't have been Obito because he wasn't even born during those times, let alone dead. So why is it, then, that Kisame identified two different faces as both being Madara Uchiha? The deformity on his face as seen on chapter 599 - it's not a deformity; it's age.
Something about the other side of Tobi's face had to have been the reason Kisame could identify Tobi, otherwise it would have been the face of someone he couldn't have ever met before. There's 2 possibilities I'm thinking of here that would explain Kisame's connection of that face to the person he met that day:
1. That other half of Obito's face is half of the entire face that Kisame saw that day.
2. That other half of Obito's face was a "deformity" in a different person's face.
Irregardless, the fact of the matter remains that the second half of Obito's face is the person Kisame identified as Madara. Now it couldn't actually be Madara himself, since that other half still exists while Madara is dead. My guess is that this person has been jumping from Uchiha body to another, and this person killed Rin to get Obito's mangekyou sharingan to activate, subsequently giving Kakashi the capacity to do so as well. The reason I mention Rin is because while Obito's death does explain Kakashi's ability to use the Mangekyou Sharingan, it doesn't explain Obito's.
I really can't imagine who or what this person is, though my best guess would be Izuna. He is the best candidate for the various knowledge and skills Tobi has, but the body shifting or what ever this Tobi person is doing to go from body to body is not like Orochimaru's. Furthermore, I don't actually think there are 2 Tobi's - I think there's only 1. I'm not exactly sure how this body shifting of Tobi's works, but I'm sure he has to be doing it given Kisame's recognition of him before Obito's time. As to why he is capable of sustaining such a high use of Kamui without going blind in that eye, I have only 1 explanation: he IS entirely blind in the eye he's been using to trigger Kamui, he's been using the sharingan covered by his mask the entire time for battle.