Directadmin + DNS

The first option is to configure both ns1 and ns2 for your hosting domain (the domain that DirectAdmin will be accessed with) to the same IP.
This will allow you to use just one IP for all websites and services. (Due note that some domain registers require two different IP (and sometimes require them to be on a diffrent subnet) for the two nameservers and thus makes this not an option in all cases.)

The second option is to have two IPs going to the same server and adding the second IP to directadmin via the IP Manager.
This will allow one server to serve the nameserver requests on both IPs but may still not work in all cases depending on the domain extentions that you have. Since some registers require to have both nameservers on a different subnet.

The third option (that is a bit more expensive but also most common) is to have two server/vpses with DirectAdmin and connect them to eachother via multi-server. This will allow you to have two different IPs for the nameservers and adds the redundency of have two servers/vpses. (and if possible in two different locations) Since DirectAdmin Multi-server will handle the transfer of the DNS records between the two servers.
And if you plan to use the second server just for the DNS records, then you can use a personal plus license since the domains are not added to the server as domains but rather as a DNS record and this do not count towards the domain limit.

But what is best for you, really depends on you and your customers needs.