One option is to use Alfresco's authentication chaining to chain more than one LDAP server for authentication purposes. What would happen in this case is that Alfresco will attempt to authenticate the user credentials with each LDAP server in order until one of the calls succeeds.
This should work with a multi-tenant Alfresco, as long as the LDAP server(s) can be configured to accept a fully-qualified userid (which is currently in the format "username@domainname", eg. john.smith@example.com).