Why not first of all set up a secondary server for all your domains and
have them get the information from the primary DNS machine on the network?
Remember, you can't have two authoritative machines as primary. When you
say "backup DNS" do you mean you want to have it turned off, sitting in the
background in case the first one fries? That would be a good reason for a
secondary server. Of course, you are right to want a backup of your