OpenLDAP Proxy for Active Directory Authentication

We run in a mixed environment, with both Active Directory and LDAP directory servers. Some users exist in both LDAP and AD, while some are just in AD. As such, we always have obstacles with password sync between directories.

Is it possible, to set up an OpenLDAP proxy (if that's the correct term), which would authenticate via Active Directory if the user exists there (or if a flag is present in the LDAP entry, etc), otherwise via LDAP if the user is not an AD user, thereby eliminating the need to store the password in both directories? Directory information would otherwise be pulled from the LDAP server, not from Active Directory.