To setup a different passphrase for FileVault and for the user (so to use your Mac two different passphrases are required):
- create a new "Disk Decrypt" user with the passphrase used for FileVault
- disable FileVault auto-login with
$ sudo defaults write /Library/Preferences/com.apple.loginwindow DisableFDEAutoLogin -bool YES
- remove the current user from the ones that could unlock FileVault with
$ sudo fdesetup remove -user USER_NAME
- make sure the "Disk Decrypt" can't administrate your Mac
That's all 🎉