Another scenario where your method will miss traffic:
A site in county A near the border between counties A and B carries traffic for agencies in both counties.
It broadcasts a transmission for a county B agency. while you have both you A and B copies of the system active.
The scanner hears the transmission from the site in county A while scanning the county A copy of the system.
Because the traffic is for a county B agency, it is ignored while scanning the county A copy of the system. You won't pick the call up until the scanner scans the county B copy of the system, and picks it up from a county B tower. If it's not broadcast on a county B tower, you miss the entire transmission. And you won't even know it.
A site in county A near the border between counties A and B carries traffic for agencies in both counties.
It broadcasts a transmission for a county B agency. while you have both you A and B copies of the system active.
The scanner hears the transmission from the site in county A while scanning the county A copy of the system.
Because the traffic is for a county B agency, it is ignored while scanning the county A copy of the system. You won't pick the call up until the scanner scans the county B copy of the system, and picks it up from a county B tower. If it's not broadcast on a county B tower, you miss the entire transmission. And you won't even know it.