Customs clearance always takes place at point of entry to Japan, so whether on one or two tickets you have to collect the luggage in Narita and recheck them. The domestic check-in area is only a few steps away from arrivals. So in your case getting the bags checked through over two tickets gives limited benefits. Only probably, that JAL could knows where you are if you are late in arrival.
That last part being said, JAL will not protect your connection on two tickets. It is your own risk. Narita is pretty efficient, so you just have to judge your gamble of on time performance of the flight.