It is an AS IT bug reported previously in this thread.

It may or may not be married but simply calling in to book both segments is unlikely to result in success because of the bug,. You can start with a call although I would probably start with an online booking and call to change it. If the KIX flight is available separately your best bet is to book the HND-LAX F flight online and call in to add the KIX-HND flight. If the KIX flight is not available separately then it is married and the strategy would be to book the J all the way through and call to switch the HND-LAX from J to F (if it can be married to a J flight it can also be married to an F flight).