You don't. You list the nameservers that you have available in your zone; the only way to really do this type of thing is via some form of unicast DNS setup. DNS resolver libraries have no concept of primary/secondary/failing over - they just know that there's a list of resolvers that ought to work. Drop me a line privately if you have questions or are interested in expanding your DNS resolvers to include more than just the two you have now.
Here are register.com's authoritative dns servers. http://www.dnsstuff.com/tools/lookup...er.com&type=NS
They share the responsibility of answering request equally.
If 1 goes down the others are there doing the same job they were doing before, maybe each will handle a few more queries.
what is "it"? you can try yourself (in a lab type envirornment) shut done 1 of the 2 name servers and query something. Though most *nix resolvers are a bit better than windows (which will fail if it attempts to query the downed server and not actually attempt another) you will see 'issues' until everything is working properly again.