So, you can see in this image the front fluid line goes to OEM distribution block for left and right front. The rear output on the distribution block a plug was added. They added a single for the switch to the rear.
That proportioning valve on the bottom is going to the rear cylinders. From my observation there is a lot of hard marketing to sell it. One of the main manufacturers of this proportioning valve recommends it for vehicles that have disc/drum, different sized wheels and any other modifications to the brake system. I absolutely don't like the knob design its way to accessible to turn it. I'd prefer a more professional set it and forget it design like double nuts.
Per forums' discussions drum/drum single to dual master cylinder conversions a proportioning valve is not needed.