You're on the right track with the second picture. . .But you'll need a couple of brackets on the frame to contact the secondary leaf pack. You can attach them wherever you want - to contact with 1" of spring compression or whatever you want. Then, when you put a load on, the second spring pack is the spring rate you are using. While driving empty, the first spring pack is the rate you are using.
Without the brackets, you would have to arch that bottom spring pack a long way before the top pack is doing any work.
Craig
