Two tickets are allowed. You may have a bit of a push back at check in, as the first airline will see you as a arriving in Japan, bring a print of your e-ticket to show the onward connection.

You can't do an overnight connection as the terminal closes, and you'd need to enter Japan when it does. If you are checking luggage you need to be certain that the first airline is willing and able to check through to the second airline. That's about it.