Board Thread:Love Live! School Idol Festival Lounge/@comment-26263841-20150602172826/@comment-24187738-20150602193809

Your device can only be logged into one account at a time. Also, there's no way to log out of an account unless you log into that account on another device, whether by Transfer Code or Google+/Apple Store integration (only available to JP for now).

What you can do is have an EN account and a JP account, since different servers run on different apps.