Sorry. That came out wrong... I think Madara lost both his eyes in the fight against the 4th, while he was controlling the 9-Tails and using his EMS to stop the 4th from Sealing the 9-Tails... Maybe Madara's eyes were pulled into the Death God also??
Whether Tobi/Madara's right eye is from Obito, IDK.... I'm just assuming thats how...
ALSO... Remember the Orochimaru was in Akatsuki at one point, Madara formed Akatsuki ... Orochimaru was able to tranfer his soul into new bodies... Hidan could live forever, Kakuzu could live forever... Zutzu is 2 people in 1. I think there is plenty of IMMORTALITY in that group to preserve Madara in some manner.
They actually talked about madara's immortality. Itachi said taking his brothers eye's madara gained true immortality