Yes you need two bind instances
When you register a domain name you provide two nameservers.
Those nameservers the registrarar will know their ip addresses.
in my example the domain is cenite.com and the nameservers are ns1.cenite.com and ns2.cenite.com
Both nameservers are running on signle debian machine. I have started two bind processes on the machine. one is binded to the ns1.cenite.com external interface and ns2.cenite.com is to the other (backup) internet interface.
Here is a tool to check your domain name
Here is example zone file
here are the configuration files.
The good think is when the first internet connection is down, the second will still serve your clients.
Bonus links
Go and switch to tinydns – people say it is much easier to configure http://cr.yp.to/djbdns/run-server-bind.html
dns zone generator