If you were following the Manga carefully you'd know that Obito has an entire room of sharingan. As for the one he lost against Konan was replaced with the rinnengan.
for dramatic effect on kishi part i guess point is he has a wall of sharingan so he had 2 in his eyes he used on for izanagi he replaced it with rinnegan but even if he didnt replace it with rinnegan he would have just gone home and poped in another one from his wall
Maybe only the eye he showed has the Kamui/dimension power and the other one was just there, just a normal Sharingan, that's why when the Kamui spins to suck something/himself up, it all revolves around one eye.
tobi hid the eye just in case he would have to use izanagi , the wall of sharingans is back up eyes in case he does use the forbidden jutsu , when he used the eye to do izanagi he didnt replace the eye with another sharingan , but with one of nagato's rinnegan .