ORIGINAL: ShootPaint
I have done exactly what you are trying to do when in desprate times to get equipment up and running but the overall results where as I listed above. I have tried it and found that no matter how hard you work at it two springs stacked on top of each other simply dont preform as well as a single spring that is design for your application. I wish you the best with your project and hope you keep us informed as to how it works for you.
Good luck.
Two springs in series is a common setup in full scale race cars. This would give a similar result in an R/C car. It is the only system that allows for two seperate spring rates to be used with one shock.