How can I have one password for FileValue 2 (pre-boot password) and another for my user login?
As you've found, there's an sync process when you change your password that automatically updates your account login at the FileVault 2 pre-boot login screen. However, if you're OK with having to login twice, there's a way to set the FileVault 2 login process on Mavericks so that you're stopped at the OS login window after logging in at the FileVault 2 login window.

The way to do this is to disable FileVault 2's automatic login, which is the process that the OS uses to automatically log you into your account after you've logged in at the FileVault 2 pre-boot login screen. Apple has a KBase article here that shows how to do this:

OS X: How to disable automatic login when FileVault is enabled

Once you've disabled automatic login, set up a new local account on your Mac and set it to use a different password from your account's password. New local accounts should be automatically enabled for FileVault 2 access, so you shouldn't need to do anything else. Since this second account is only going to be used as a FileVault 2 login, I'd recommend making it a standard user with no admin privileges.

After that, reboot your Mac and login at the FileVault 2 pre-boot login screen using your new account's password. When the OS unlocks and boots, you should then be stopped at the OS login window. At that point, login with your regular username and password.