It has been that way for a number of years (my other half is a former dispatcher there, and she remembers the two channels going in between 1986 and 1990). Most likely it was due to a lack of available VHF or UHF frequencies at the time.
Going to 800 allowed them to simulcast the entire county and add command and tactical channels.
The "old" Control 9 EMS channel (UHF) is now used for the voice dispatch paging. It used to have 3 primary tones (Copernicus, Rodoni and Holiday Lake), but I believe it is also now simulcast.