This is a hypothetical question.

Let's suppose I have two networks, each with its own DNS server. Network A has names a1.local, a2.local, ... and network B has b1.local, b2.local, ....

Zone file for each of the networks looks something like this:

$ORIGIN local
@ IN SOA .... blah blah blah
a1    A 1.2.3.4
a2    A 2.3.4.5
... 

for A, and

$ORIGIN local
@ IN SOA .... blah blah blah
b1    A 3.4.5.6
b2    A 4.5.6.7
... 

for B.

Now I also have a regular internet domain example.com and I want to access the machines as a1.A.example.com, b1.B.example.com, ...

How will I have to change the configuration of name servers in networks A and B?

(in fact I am writing a super-magic DNS server, currently serving A and B separately, but there is a chance that I will have to add the ability to merge the networks; so I'm interested in knowing the problems which lie ahead of me and how to prepare for the possibility)