Why the sum of the responses of s and s increase with 20dB per decade or 40dB per decade?
If s = 1 and then changes to 10, the square of s changes from 1 to 100. So for a change in s of 10 (1 to 10 or a decade), the amplitude changes by 100.

Now convert 100 to decibels by taking the log and multiplying by 20. That decibel change is 40 dB in one decade.