They are two different sites that act as backups to each other over the same geographic area. The main thing is that it spreads the load between the two sites. Fort Worth Police are and some public works users are on Layer 1 along with Law Interoperability(IO) channels. Layer 2 is for Fort Worth Fire and and most of the other area fire and police departments that use the system in that area, as well as Fire IO channels. There are exceptions, but that's a quick summary.
Dallas is just now getting close to joining the system, and will likely have most city public safety stuff on their site 3-1 with city public works and county stuff on 3-2.