That part is obvious but why did it happen? I know killing your best friend is considered the best way to release the ability to use mangekyo and if the eyes are linked could it have then released mangekyo for both of them at the same time? And the sharing of the vision is a strange side effect involved since the eyes linked but in different people? I don't see an ability ONLY being unlocked by killing your very best friend no matter what. I am willing to bet mangekyo is unlocked though emotion and killing your best friend is the best bet... but killing a very close comrade could also do it. Plus by that point in time rin could be kakashi's best friend. And I know she is not dead yet but she technically is moments away from being dead.
I am just trying to understand why we have NEVER seen the sharingan share vision like this ever before? It has to be because of the fact that Kakashi just killed or mortally wounded someone so close to him.