It all boils down to your personal risk tolerance. By separate PNR's, I presume that you mean separate tickets as well. If so, is there a reason not to book this travel on one ticket as BA and NH have an interline ticketing agreement?
If there is a good reason, I would definitely allow an overnight at FRA as it is not only about checking in at FRA, but about collecting bags, Customs and then dragging oneself to NH to start a second journey. Not just weather, but ATC and any little thing can turn into a problem for you.