Denis Fondras wrote:

...

What? It's a typical configuration with glues.

For example, in my organization, ns1.noc.titech.ac.jp is the primary for noc.titech.ac.jp and titech.ac.jp.

Sorry, you are right, I probably haven't understood.

A more artificial configuration is primary1.childzone.parentzone.example.com is the primary for parentzone.example.com and primary2.childzone.parentzone.example.com is the primary for childzone.parentzone.example.com But, note that DNS operators in the real world are so creative. Masataka Ohta