After you have split the database, open backend in 'exclusive' mode, use File tab to add password. Close backend. Open front-end in 'exclusive' mode. Use File tab to add password. With front-end still open, delete links to backend file(s). Use External Data tab to re-link to back-end file(s). On this occasion, you will be asked to supply the password for the backend file(s). Do that and then close front-end. Next time you open front-end, you will be asked to supply password for front-end but you will not be asked to then supply the password for the back-end file(s).