Is nobody else shocked that buying a ticket on Delta.com with an alliance partner could result in two
separate PNRs with two separate tickets? And we're absolutely sure this isn't the case of having two PNRs because one airline is on Sabre and the other Apollo (or whichever, I don't know what DL or MU use)?
Certainly in that case, even if you can't OLCI with MU, there should be no issue checking in for both flights at ATL.
I'm kind of flabbergasted DL would sell a connecting itinerary as two separate reservations/tickets.
ETA: OK, I see now that at least one other person is shocked. 