Ah, the stories of lack of interoperability...In any case, I took a look at the database and it appears that your County and City have some kind of patched talkgroup together. If you look at Clarksville Public Safety Trunking System
, you'll see TG 788 which is described as Patch To Montgomery County SO On TN Homeland Security Trunked System
. I'm guessing this is to allow them to talk to each other in big events.
Where I live, there are a bunch of different systems that people use ranging from VHF to EDACS to P25, and we have a bunch of patched frequencies/talkgroups together so people can talk to each other. I wouldn't know too much about this setup as I don't live in Tennessee, but maybe the good people on the Tennessee forums have an answer for you. Hope I helped in some way!