Just as an FYI, here's what I got from the Delta Travel Center...............

"In response to your inquiry, if all the connecting flight data is in one single record, you would be able to check the luggage through. If the flight data is in two records, you would have to claim and recheck your luggage. "


So, in essence, if you list for the DL/NW flight and then list for the other, it is the same as an interline flight........the two records are separate and are treated as such.