I know this is possible.
When I first installed Mavericks, I set up a default password of "password".
I then enabled Filevault 2.
At some point I then changed my password from "password" to my actual password.
However the pre-boot password is still "password".
I like the fact that there were two different passwords, but I want to change the pre-boot password to something better than "password" (obviously!) but...
On my laptop, I've done some testing and changing the user login password also updates the pre-boot password.
And if I fail the pre-boot password 3 times and use the recovery key, I can update the password, but it only updates the user login password, which in turn automatically updates the pre-boot password.
So they are always in sync.
So how on earth did I originally have 2 different passwords, and how can I continue to have 2 different passwords?