How have you configured high-availability with standard Mail Server on two NAS, with one static external IP address? Using a dynamic IP address should be very similar, not that I understand how you have this working in the first place.
Some details on how you have setup the two mail servers would be helpful.
When I changed from using local user accounts to LDAP I created a duplicate set of accounts in LDAP. Then I copied the .MailDir folders from the local account to the corresponding LDAP account and applied changes to the folder's owner and permissions (and it's contents). This might be something you might think is similar to what you could do, if the two NAS run separate communities of users: create extra local/LDAP/AD accounts and copy in .MailDir.