However all digital content can be played through any profile as long as the both profiles are on your HDD which leaves little room for concern. Now the only major problem I have run across is that some games require Microsoft points to buy them and american cards will not work with the account so I learned a lesson there. However if you have credit card/debit card then you should be fine. The biggest hurdle is making the separate account so follow the directions to a letter from this site. So will you be making a Japanese x-box live account now??
-Cecil The Dark Knight