Ya know...it's still confusing even to me. When working a NW flight that has a DL codeshare number...it seems to me that the bump compensation should be given in the currency of the actual airline that operates the flight. But, I've seen some of my compatriots give compensation in DD on NW operated flights because it's easier to comp in the Delta system. I guess that's a question I'll have to ask when I get back to work on Thursday.
To try and answer the question....as of now...I don't think the DD voucher will be usable on NW.