[NTLUG:Discuss] "Back up dns server"?

[m m]

All:
 
We all know that we need to have two DNS server for a domain.Usually, these two DNS server have the same A records for a domain.All of this settings are preventing the web site (mail...) failure.  
 
Based on the concept above, I think we should be able to set the 2nd DNS server to a different IP with same domain name as primary DNS server.
In this case, there is a need to run 2 servers with same domain but different IP. 
Why is doing this?  When one of the IP is not available, another one will be still available.
 
Please note, this is not a web farm issue. I think this will be to fix IP (or firewall) "broken" issue.  
 
Am I right? Any company doing such a "back up DNS server"?
 
Thanks.
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