@vasilis:
If you're not running DirectAdmin you're in the wrong place.
But I'll try to give you some generic help.
Are you running BIND? If you're not running DirectAdmin, then we cant help you with that; if you are, then you probably are running BIND. And also, if you are, then each time you add a domain, DNS will be set up automatically. So we don't help you with that, either.
Are you trying to set up two IP#s so you can register a domain to use your nameservers? If so then you really should set up BIND on two different servers, but if you can't or won't, then you should have at least two IP#s on your server where BIND is installed.
Choose a domain name you own to use for your namesevers. For example, example.com.
Then at the registrar where youve registered your domain, set up two nameservers, with those two IP#s.
That's NOT the same as assigning nameservers, and we can't tell you how to do it because it differs from registrar to registrar. But for example, ns1.example.com, and ns2.example.com.
Once you've done that, then on the server(s) hosting those two IP#s, install a zone file for the domain, also with NS records pointing to those two nameservers.
If you need to learn how to run BIND, then find a forum or site which teaches that. It won't be us.
Jeff