How to add a new nameserver in whm?

How can I add a new nameserver in whm? My main domain is www.mymaindomain.com.

I have another account in my server with a different domain like www.accountdomain.com.

As of now for the two account I am using the same name servers

www.mymaindomain.com www.accountdomain.com

ns1.mymaindomain.com ns1.mymaindomain.com
ns2.mymaindomain.com ns2.mymaindomain.com

I want to createanother name server in whm and changed my accountdomain.com's name server into
ns1.accountdomain.com and ns2.accountdomain.com

I greatly appreciate your help.