Loading effect of two stages of RC filter
Your equation is for two ideal low-pass filters in series. The resistor/capacitor realisation is not ideal you have a finite input and output impedance.

To get a better approximation of the ideal behavior you have to make the value of the resistor in the second stage larger compared to the resistor in the first stage. Don't forget to scale the capacitor accordingly.

Alternatively you could add a buffer to the output of the first filter, just to see the difference.