Requiring two IP#s is an oversimplistic solution to the requirement that there be two different nameservers for each domain.
If in fact you use your one physical server, with two IP#s, and use both those IP#s for your nameserver entries you lose all nameserver redundancy. I know some people think that's unimportant, and to you, it may be, in which case use DirectAdmin with two IP#s, and put each nameserver on one. Beware, though, that some DNS reporting tools will catch this and show that the DNS is not redundant.
You may be able to get away with one IP# and use it for both nameservers, but some TLDs won't allow you to register domains if both nameservers you assign are on the same IP#.
In that case you may need external slave DNS. We, and others, offer SlaveDNS. It may be more expensive than just getting another IP# for your server but it will give you real DNS redundancy instead of just the illusion of DNS redundancy.
Search the advertising section of these forums for entries for Slave DNS.
Jeff