Plotholes

[AaaaNinja]

Because their both the same jutsu just works a little differently.

I think you don't really understand the issue. It's true that Obito and Kakashi have the same jutsu, but Kakashi had no idea of the existence of another eye so as far as he was concerned he discovered the jutsu. Somehow he happened to give it the same name.

I think that people who ask the question of "How did it end up that both of them named it Kamui?" forget that before his identity was revealed, Obito always called it "space-time jutsu". Now that everybody concerned knows that it's the same jutsu and from the same pair of eyes, Obito adopted the name by calling his eye the real kamui, and he's been calling it kamui ever since.