Iq believe Obito is going to send Sakura to the real world right before Madra can kill them both. Of course Sakura doesn't destroy the eye because she's worthless and it would make it 2 easy for naruto and sasuke to fight a one rinnegan Madra. I believe Obito will use the last of his chakra to send her back, making him unable to suppress black zetsu any longer. Then Madra will get the other rinnegan, and give black zetsu Kakashi's sharingan. Why would Madra throw one away when he can keep it and have a pair. After that black zetsu kaumi's them back to real world. No clue what happens after that. This is just what I think will happen with the eyes.