As far as I know they use the second when they have units on a scene and want to "hold other traffic on that channel." They move the other units to channel 2 and have the on scene units continue on channel 1. Dispatch will then be on channel 2 until the situation on channel 1 is cleared.
Most other agencies I have monitored move the "on scene" units to a Tac channel and continue normal dispatch on the primary channel.
I have not done much monitoring ot FCSO in the last couple of years, things may have changed.