domain name servers and redundancy

This question has been asked before, but none of the answers really seemed conclusive so I thought that I would bring the subject up again.
Here is the scenario:
I am a reseller hosting with one company.
I have ns.mynsdomain.com and ns2.mynsdomain.com.
Each of my client's domain records lists my nameservers (as opposed to the actual host's dns)
I have one client in particular that is more or less mission critical. So, I want to open a second account with a sererate host with a different NOC and use ns3mynsdomin.com and ns4.mynsdomain.com. The e-mail issue I would assume would be minor since this clients e-mail is mx'd over to a third party mail server.
By doing this can i increase uptime and insure against failure? I am assuming that if ns1 and ns2 crash and burn that ns3 and ns4 can pick up the slack.
Any comments, references or suggestions would be helpful.

 

 

 

 

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