Best is to have seperate geologic locations for both nameservers. We stopped using the local ip's only on 1 server and switched the ip's amongst the servers in the nameserver records to easily achieve this.
For example on server 1 you have 2 ip addresses, I will use LAN ip addresses just as example:
192.168.10.1
192.168.10.2
with domain example.com
And on server 2 you have other addresses:
10.0.0.1
10.0.0.2
with domainname foo.com
Now what we do to have really external nameservers on different locations and subnets, we exchange those, so we set it up like this.
On server 1:
192.168.10.1 ns1.example.com
192.168.10.2 ns2.foo.com
On server 2:
10.0.0.1 ns1.foo.com
10.0.0.2 ns2.example.com
This is quite easy to do, as you only have to specify the correct ip addresses at the ns2 A records on both servers.
So in example.com on server 1 it will be:
ns1.example.com. 3600 IN A 192.168.10.1
ns2.example.com. 3600 IN A 10.0.0.2
And you do th same on server 2 with the other ip's.
ns1.foo.com. 3600 IN A 10.0.0.1
ns2.foo.com. 3600 IN A 192.168.10.2
Ofcourse, these ip's must also be setup at your registrar as ns1 and ns2 for your domains.
This way you have a correct redundant dns. If server 1 goes down, server 2 will still as secondary nameserver due to the multi server setup and for example mailservers will know domains exists but are only down.
Same if server 2 will go down, on server 1 the domain names will still be presented as existing, but just not reachale for this moment.
Is this what you were looking for?
It's always possible to add other ip's like this from other servers for ns3 and ns4 if you really want that.
