HM> you cannot give a built-in way to use
HM> TWO NAMESPACES without a forwarder.

Neither he nor you can give a way to use two namespaces _with_ forwarding,
either. There's no way to use two namespaces, full stop. As I said, (and
with the given that we are restricting the discussion to the "IN" class) every
entity making use of DNS sees exactly _one_ DNS namespace.

HM> Two namespaces require a forwarder because a single DNS server
HM> can only check ONE ROOT and the namespace it anchors. Clients
HM> only query one server (the one that responds to them.) To query
HM> another namespace you MUST use a forwarder that is attached to
HM> the second namespace through another root.

The notion here of querying a second DNS namespace simply makes no sense. The
only way that one re-combines data sources in DNS, overriding what the
delegations do, is by picking some point in the namespace tree and specifying
that for all domain names on one side of that point data are fetched from the
public DNS database and that for the other domain names data are fetched from
a private DNS database. This doesn't provide two namespaces. The DNS
database content that one "sees" may comprise private and public content
obtained from many sources stiched together in a complex patchwork, but
there's still just the one namespace for the owner names of the resource
records.