I'm assuming that you are talking about Linux LDAP clients here.
I don't think that there is a native command to keep them in sync (you could write a script to sync periodically, but that can be messy and you would need to use the same one-way hashing mechanism). What I would suggest doing is to build a wrapper script around the real passwd command and the ldappasswd command that takes the user's password from stdin (without echo'ing it) and then running the appropriate commands using the value that they provide.
You could even name the command passwd that is located earlier in $PATH or set up an alias that applies to all users' shell profile.
