There is no official position of Majority or Minority leader in the Senate. The last time (2000) we had a 50/50 split the leaders of the Senate negotiated an agreement on how to proceed. Trent Lott was Majority Leader by virtue of a Republican VP. More important committees were split 50/50. That's a far more important consideration and I don't think there's a Senate rule addressing that.
So what your saying then is Mitch will get Chuck’s coffee in the morning?
I wouldn’t be surprised if the Rats, in a mainly symbolic “healing” gesture allow Mitch to be Majority Leader, even though for all intents and purposes it would be meaningless.