Okay, here's my theory. An Uchiha can't use Sharingan until they awaken it. Afterwards, they know what the chkra flow of the Sharingan is and use it whenever they want. My question is, can Kakashi mimic the chakra flow in his Sharingan eye in his original eye? If so, that would mean that Kakashi has 2 Sharingan at his disposal, and the new one may or may not use a lot of chakra.
Sidenote: If this were true, I'd love to see Kakashi shoot an MS laser from his eye!