Well, AFAIK that's not forbidden: "external" clients get pointers to the default ns/ns2 and "internal" clients may use the others, less stressed NS [in addition to the defaults].
It seems that simple.
But it has to jive.
Outside DNS systems must all agree as to which DNS servers are authoritative.
The "tree" can't say it's branches "A, B, C".
Then you ask branch "B", and it says branches "A, B, C, & D".