Re: Secret behind Obito's Kamui?
both of obito's eyes still likely share a connection. i think kakashi's MS also activated when Obito's was activated (the shared vision could be a hint) but it was only during after the timeskip, when kakashi continuously trained and improved his chakra capacity, that he was able to pull it off. kamui was shown to consume a lot of chakra, and kakashi as a kid even if he had the kamui probably won't be able to use it anyway.
Obito can use it continuously because he's attached to a hashirama clone that could retard the degradation of his sharingan, and would also act as a decent chakra source IMHO.