A reseller client of mine wants his own DNS, but it should be pointing to the same IPs as mine.

How do i do this?

Im running CPanel/WHM on RedHat9

There really is no configuration needed on your part. Just have him add the A records and use those IPs at his registrar.

You can add 100000 DNS on 2 Ip's
first do the following ..
1.Login to Domain Control Panel and then add there
ns1.domainname.com
IP:Main Server IP
ns2.domainname.com
IP:Second Server IP
After 24 - 48 hours it will be actived and ready to use but first you need to do the following in WHM
Login to it
go Edit Setup ..
In NameServers
but ns1.dominname.com and then press at Add an A entry for this nameserver and do assign for it in the POPUP then do the same to ns2.domainname.com and do not save Edit setup ! just exit ... after that give the DNS to you reseller it will be fine
Regards