There is no current option to fix the problem of having two password. However, what you are asking for will give rise to the user being unable to get email if external LDAP goes down. To get round tha you might want to set the following:
Code:
zmprov md <domain> zimbraAuthFallbackToLocal TRUE
which will fallback to the zimbra ldap for authentication. If you consider the current set-up a problem then vote on this
bug.