PETER:
I can only present a basic idea, since the design, I did, was for a commercial entity and I have to respect the contract.
The isolated supply is to drive the power Mosfets and the isolation could be just above the controller positive supply rail if the voltage of the isolated is just at their supply level, like two 12 batteries that need isolation when a vehicle is OFF and when it is ON and battery charging to be able to charge the isolated battery with the same voltage level of the other battery.
Full isolation, if the voltage of the controller is different from the batteries being switched.
The Controller can be a simple circuit like an optocoupler driving the small P & N Mosfet gate drivers or a regular gate driver used with the power Mosfets.
The power Mosfets in parallel must have very low Rdson to insure very low losses when ON, like a number of Mosfet that all of them in parallel-series produce 0.001 Ohms Rdson, this implies that with this low value , 100 amps can be switched, since the voltage dropped across the Fets is just 100 millivolts, representing 10 watts dissipation across the Fets.
Temperature behavior has to be analyzed to insure proper low saturation voltages.
Nando