Obito's two eyes have to be identical, especially given that they both went Mangekyō at pretty much the exact same time. The difference between Obito's and Kakashi's abilities must have to do with what they each figured out about their eyes. Obito figured out early on that he can use Kamui on himself in more ways than one. So he never felt the need to train its range (always being able to get close to an opponent). Apart from Hashirama's cells, this should also be a reason why using Kamui doesn't drain his chakra. Kakashi on the other hand, well...

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