[Driven5]

The circuits are not separated front and rear, they are separated diagonally LF/RR and RF/LR. (Edit: No they're not...See Post #32)

Correspondingly, stepped bore (or quick take up) master cylinders do not have each bore feeding a different circuit. Rather, the large piston moves more fluid to take up the caliper piston retraction and build the initial pressure with less pedal travel, then the small bore pistons take over the high pressure use. Thus the brake balance equations should be using only the smaller bore diameter on both front and rear. (Edit: Maybe...But maybe the equations should only be using the larger bore instead. See Posts #32)

So if two master cylinders have the same size small bore, they will feel the same under moderate to severe braking. However, the one with the larger large bore with feel firmer and travel less initially. As the size and quantity (total area) of caliper pistons increases, so to does the need to increase the size of the larger bore.