Several people already mentioned this. When someone is sealed with the reaper death seal the user and the object(s) that are sealed spend eternity together in the death reaper's stomach. Most of the time, they are enemies and spend it fighting. At the same time it is possible for them to solve their problems, since well, it is eternity. Minato was in there for 17 years, and he was considered a genius, and has been shown to be very skilled with chakra manipulation. He could have coerced and trained with the other Kurama for quite a while. Obito, I don't really have a good explanation for. Even with new powers one just can't understand all ninjutsu like that. You can have the ability to, but not just know it all of a sudden. That part makes no sense. The taming part may, since the Juubi doesn't have much of a consciousness, you would only have to overpower it, and that's it. Again the Minato part can make sense, and I don't find much sense for Obito at all. Take my response as you wish.