Valve springs

[adsnape]

Im replacing the valve springs on my 1.9dg, is it ok to swap inner and outer springs on my head to a single heavy duty set? What's the difference? And why two sets? Is the engine less responsive with a single heavy duty set of springs?

[Rosie n' Jim]

As I understand it, valve springs have a resonance, causing the valves to bounce off the valve seat at a specific rev range. Using two springs of different size and thus resonance  overcomes this issue. A bigger stronger spring would have a very high frequency resonance,  way above usual engine rev range. 
This is important in a race tuned engine, probably less so for our low revving units.