Have you looked at booking NW to KOA instead of to HNL?
I think you may be able to get an all-in price that's less than NW to HNL & HA to KOA on separate tickets.

With respect to your original question - I'd agree with the other responses you've gotten. There's also a fair chance that NW or HA may make schedule changes between now & Feb --- if you've got a tkt from NW through to KOA, you've got better protection if schedules change.