I believe that the correct way to achieve integration with multiple directory services would be through leveraging a unified/federated login service like Centrify. That will allow ONE service to control authentication and consult the other service when needed. One of the problems that I could foresee with joining a filer to an AD domain and also specifying a second LDAP directory service would be conflicts with permissions. Users with accounts in each of the different directories that have conflicting permissions could cause a security issue (not to mention, there's no mechanism that I'm aware of for the filer to decide which is authoritative). If a single sign on service isn't an option, then I'd suggest what Radek said above and look at vFilers for this. You can have one vFiler joined to one domain/LDAP and others joined to different ones and present no issues. Hope this helps.
Sorry for the late response, I'm not very often on NOW.
Yes it definitly can.
I have a Metro Cluster and some standalone boxes and all are AD integrated for CIFS and LDAP (openldap on Linux) integrated for NFS. And there is even a facility to map AD users to LDAP users (e.g. globalAadmin@Company.domain <=> sysadmin (uid=1234, gid=4321)) for multi-protocol volumes.