I would assume that its possible for a Sharingan user to copy another Sharingan user's technique, even the Mangekyo. Lets say that the activation of the Mangekyo requires chakra to be concentrated on the eyes, and specific parts of the brain (for the images that the brain wanted to display to the target). If a normal person tried this, even knowing the exact amounts of chakra needed and such, it would do nothing simply because they do not possess the Sharingan in the first place. What would happen if someone tried it who did possess the Sharingan? Also, given the Sharingan's abilities to see chakra and its obvious abilities to view it in such detail to allow the Sharingan user to copy the technique, would it really be impossible to Kakashi to atleast mimic the Mangekyo? I'm not saying Kakashi has it, but if someone like Sasuke were to see Itachi perform it, why wouldn't Sasuke be able to do it too?