In serverA, add 'A record' in for 'sub', 'ns1.sub' and 'ns2.sub' under 'mydomain.com' dns setting. Also 'NS record' for 'sub' with refer to 'ns1.sub.mydomain.com' and 'ns2.sub.mydomain.com'.
In serverB, create dns for 'sub.mydomain.com' with 'A record' to 'ns1' and 'ns2' and 'NS record' for this ns.
Yes, the logic is that true.
But I mean in this case, in Kloxo server B (hosting for subdomain), in the domain Tab :
Do I have to create "mydomain" or "sub.mydomain" first ?
Bcs If I create "mydomain" first , DNS record will auto mapping "mydomain" - y.y.y.y
If I create "sub.mydomain" first, its did not appear my main domain for adding