I'm pretty sure they use the motorola centracom software at greenville county. We used that at the agency I used to work for, and what probably happened (what you heard the other day) is they had both of those channels "highlighted" on the screen, and then were either transmitting by pushing the red "general transmit" bar on the front of the CIE, or by using the footswitch. When you transmit by either of those methods, you broadcast over whatever channels you have highlighted.
If you have multiple channels highlighted and only want to transmit on one, you just have to point and click.
That would be my best guess! The above happened a lot where I used to work. The red/general transmit bar is a lot more convenient to hit as opposed to having to point and click every time, but you did have to be careful to watch what channels you had selected.
That was probably the situation. Since he had GC EMS and Berea Fire department on the same scene, he had both on. I have heard GC EMS out with county FD's that must have been on VHF and didn't note the different arraignments.