Go look at the house batteries and see if they are in series rather than parallel. See attached image.
To get the 24V they must be in series. I doubt that you have a helm switch that creates a temporary series. While it can be done, usually the parallel switch allows you to draw additional ampage of the same voltage if your batteries are a bit low - mainly to start your engines. I have never heard of it being used to create 24V.
Now it could be that the rest of your system is 12V and the stabilizers are the only things pulling 24V. Then it could make sense to have a series switch, but I am wondering how they isolate the 12V equipment?
As said, look around the boat to see if there is a wiring diagram, or an owner's manual that may contain the same.
Connecting Batteries in Series or Parallel