2003 AD design - County Government/Public Safety

I'm looking for anyone who may have some advice or may have had
experience with this type of situation in the local governmetn/public
safety sector.

We are currently working to develop and migrate from NT to 2003 server
and exchange. The actual migration part of this is not a problem
however the question of what is the best way to setup the network has
become an issue. We have two IT groups, one for the county gov and one
for public safety, each at their respective locations. The question
has come down to does it make
more sense to create an AD structure with a county gov as the only
domain and all other departments, including public safety(which
includes dispatching, fire, EMS, Emergency management, 24/7 operation)
as OU's or would it be beneficial for the public safety division to be
another/separate domain within the AD structure??? Currently the two
sides manage their own servers, exchange, network and have minimal
interaction with the exception of email.

Any thoughts, ideas, suggestions, feedback...etc are greatly
appreciated.

Thanks...