Two NH tickets you will get both boarding passes at IAH.
NH-TG with 2 hours at NRT should be no problem either way. If you have status with NH they will usually try to check you through; same if you are in Business. In economy they might not, but even then I think 2.15 hrs at Narita is not a problem. Never spent more than 30 min in security, and if you do, your bag for sure will be waiting on the other side.
It will always help if the two tickets are on one PNR so the check-in agent can simply check you through without having to combine two PNRs. I have done this i.e. on BRU-NRT-PUS (NH/JL); on AKL-NRT-PUS (NZ/KE) and SIN-NRT-PUS (SQ/KE).