have 2 or more vps

hello
my friend ask me to solved this


he have 3 vps with 3 public ip
vps A:1.1.1.1
vps B:2.2.2.2
vps C:3.3.3.3




the primary domain - abc.com
he set private /child ns to

ns1.abc.com:1.1.1.1
ns2.abs.com:2.2.2.2
ns3.abc.com:3.3.3.3

and the abc.com use NS




ns1.abc.com
ns2.abc.com
ns3.abc.com
so far i think all is good... but.. the question comming out
how if he want to add other.com to vps A only and other.net to use VPS B only what ns he should to use?


temporary i use this solution for him
create NS1 and ns2 pointed to 1.1.1.1
ns3 and ns4 pointed to 2.2.2.2
ns5 and ns6 pointed to 3.3.3.3


and his abc.com use (whmcs only allow 5 ns)
ns1.abc.com
ns3.abc.com
ns4.abc.com
ns5.abc.com
ns6.abc.com


and add dns zone on every vps and set abc.com IN A 1.1.1.1. and i ask him to build abc.com at server A.


back to my question how if he want to add other.com to vps A only and other.net to use VPS B only what ns he should to use?

Are the NS on different subnets? Otherwise, far too easy to get DDoS'ed...