Different sub domains can be directed to different IPs servers but a client server cannot be forced to keep looking at name servers for additional records.
To delegate a subdomain to a zone on another server, one would need to add NS-records for the sub-domain name. This would point to the host names of the DNS servers hosting the sub-domain - in the parent zone.
In the "DNS records" window right-click the parent zone in the left list and select "New NS-record":
