There is nothing that prevents you from using one IP address for two nameservers. This is not an ideal situation, since nameservers should preferably not only have different IP addresses but be geographically separate, but there is nothing stopping you from doing it.
Note that
Main >> DNS Functions >> Nameserver IPs does not directly affect your nameserver configuration. It is provided for your reference only. If this is what you are asking about, it does not affect the operation of your nameservers, and you can host nameservers successfully without ever entering anything in this page.
To configure your nameservers, simply edit the DNS zone for the domain, using
Main >> DNS Functions >> Edit DNS Zone, and add NS and A records for the nameserver hostnames. That is all that is required. Once this is done, register your nameservers' IP addresses at the domain registrar as nameservers. Once these two steps are done, your nameservers will begin to work.
We have comprehensive documentation for the WebHost Manager and cPanel available online. The index is located here:
All Documentation
The documentation should help you, but please feel free to ask if you still have any specific questions.
