Here is a good resource for spring information, and to learn about how the work.

http://www.afcoracing.com/tech_pages/spring.shtml

Alot of great info here.

I know its RC cars, but its interesting to figure things out. I come from, and still set up and work on Dirt Late Models, so alot of this i researched for a reason.

Stacked Spring rates, very popular right now in DLM's:

http://www.jimmyjudd.com/jimmy/spring_rate_cal.htm

Only after one spring can not compress anymore, or its movement stops does the other springs rate takes over. Most off road application, the shorter spring has a limited amount of movement, once it hits a stop the spring that is active takes over the spring rate.