Each district is assigned 2 or more Dispatch channels. Depending on activity and need they may run 2 dispatchers and assign certain ops to each channel.
Usually, with one dispatcher, they just use the A channel, but during busy times they may use both. If they have standing procedures for 2 dispatchers and channels they will often patch the two when a single dispatcher is working so that field units can remain on their regular channels.
As for the specifics of District 8 I am not sure. With the recent and ongoing consolidation of ISP communications things are going to change. I would not be surprised to see different districts be simulcast with each other, especially after hours.