Unless the FD has monkeyed with the wiring (like that has NEVER happened before -- you'd be the first), it's not dual-voltage, just straight 24 volts. 2 @12v batteries mount in a box between teh seats.

They may have added a second battery to run things like the blinky-blinky fire lights, siren, pump starter, etc. Or just pulled 12v off of one of the two series-wired batteries.