You do not need two nameservers. 

Do not replace your current nameservers or you will be in a world of hurt.

Get this:

Your knee bone (whois record with the internic) is connected to the leg bone (your primary name servers). 

ADD ONLY one server, or your updates and pulling secondary primary records will break.  If you don't yet know how to pull down your zone records, or what recursion means or forwarding refers to, heed my warning. 

Believe me, you could study for a 1 year and still not know enough to take on maintaining your DNS via bind.  I know I have done it for 25 years. 

Simply add your server to the list - ONE SERVER.

DNS is heavily hacked, be warned, as you open the ports, and be careful to protect your rdns keys.