That's interesting.
Does anyone have any experience with this?
Or any staff know for sure?
Would adding an SM number to a return leg automatically credit the outbound as well?
That would be annoying.
I have found that once you check in for the outbound that DL cant change the number or take it out.
If you plan on no number on the OB and entering it for the return my fear is that even delaying the entry until after you fly the OB Delta might give you credit in both directions anyway. I am not sure on that but I would not want to test it if I really didnt want the miles in both directions.