Two masters for one zone

[Herb Martin]

You could use a Win2000+ DNS server running
on a Domain Controller (perhaps a dedicated domain
just for this purpose) provide the two or more "Masters".

It's not BIND but if you need it, there is is.  BIND secondaries
are supported.

Technically their called "Active Directory Integrated DNS
Servers" but as a 'set' they play the role of Primary DNS.

How hard would it be to add multi-mastering to BIND9?
(I have been thinking about it anyway...)