I consider one hour to be realistically sufficient for NW to NW connections at NRT, especially since an on time departure of the TPAC and longer intra-Asian flights usually means than they arrive into NRT about an hour early. Passengers with tight connections are usually herded into shorter security lines at NRT. However, the gate layout could be confusing to a first-time traveler.

You should have no trouble checking your luggage through so that you don't need to see it at NRT but be sure to check that the bag is property tagged before it goes away. Try to do OLCI for both flights before going to the airport and be sure to bring paper copies of the details of both tickets with you.

It might be worth calling NW before the start of your trip to have them cross-reference the tickets in both records. If you do this, be sure that you are talking with a NW agent, not DL. Hang up and call back if needed.

In late December, the added security at NRT occurred at the gate (in particular, in the jetway), but flights did not appear to begin to board early.

The risk with the two separate tickets is that if one flght is late and you miss the other, NW is technically not responsible as they would be if it were a single ticket, but since it's the same carrier, I would expect that they would nevertheless find another flight for you, probably the following day.