My assumption is that these slots are only used for competitive advantage, so four on each side would probably be used for the routes with the toughest competition between the JVs:
4x JL = LAX, ORD, SEA, HNL
4x NH = SFO, LAX, SEA, HNL
After that it's anyone's guess. I think fortress routes like DFW-NRT and IAD-NRT (only served by one of the JVs) would stay at NRT unless the other JV tries to start them out of HND, which seems like a distinct possibility.
JAL might try to move multiple HNL flights to HND as a means of competing with the ANA A380 service.
A new JAL route to MIA or PHX would most likely be out of NRT since they wouldn't be competing with anyone who runs it nonstop out of HND.