Nameservers

Hi new hear and new at using this system.

I have 2 domains and I had one changed to the nameserver with no problem.

I have tried changing the second one with no success, my question as I'm not sure. the nameserver given when I purchased this system. Is that used for all sites I attach to my program?

I hope you understand.

The name servers worked for the first site, but not the second do I need a different address?