Contrary to what one would think the two spring rates become cumalative when the springs are acting one against the other. This gives a very high initial spring rate but when the lower spring is freed, either in bump or due to cornering forces, the rate suddenly falls. This is what gives rise to the nodding dog syndrome.
I think that is part of SteveH & Harry's car's problem. When I had a look (admittedly passenger side while Harry was sitting in the car) there was no freedom on the rebound spring. I suspect more miles and resetting the suspension again may sort the nodding dog that is sometimes experienced.
