Not sure if NH will check your bags on separate tickets, but with such long layover I would not worry too much.
Are you sure you can not get the same flights on one ticket? I looked up random ITM-NRT flight and the the fare allowed END-ON-END combinations with Delta. If both NH and DL fares allow END-ON-END, you can get the same price on a single ticket (plus/minus taxes that maybe calculated differeny) You will need to call Delta or use multi-city option with OTA to get it ticketed.