i have ns1.domainA.com point to an IP

i want to share that ip with ns1.domainB.com.

how do i do that?

i have registered ns1.domainB.com and point it to the IP

but the site domainB.com is not pointing to the IP.

why?

is there something i need to do in WHM?

i only add ns1.domainA.com in WHM.

when i try to add ns1.domainB.com in WHM, they said i have used up all available IPs, even though i have selected to 'allow nameservers sharing ips'