How to know exact Decoupling Capacitor values for supply voltages?
Short answer: 

They start with the standard (0.1 uF in parallel with 10 uF) and if they still have supply bounce issues they start experimenting until it works.

Longer answer:

The exact value of the decoupling capacitor isn't critical, typically, unless you already know something about the frequency components of the noise you expect to see.

Typically, the smaller the capacitor, the higher the self-resonance frequency (the frequency at which the device stops acting like a capacitor and starts acting like an inductor due to internal phase shift).  That is why datasheets typically show multiple devices in parallel.  The 10 uF is to bypass lower frequencies while the smaller capacitor is effective at higher frequencies (and less effective at lower frequencies because it is smaller).

When you bringing up a new chip on a test board, the product engineers where I work typically use a standard decoupling of 0.1 uF in parallel with 10 uF as you say.  If that is good enough, then great, they're done.  If there are still supply bounce issues then they will experiment (analysis and simulation is not super helpful here) until they get the noise under control.  They do this by adding smaller caps very close to the chip or sometimes, like you saw, parallel 0.1 uF caps.  This works because different 0.1 uF caps are different distances from the chip so they interact with the supply bounce in different ways (and at different frequencies).