Electronic – Effect of using buck converters in parallel

But a Low Pass filter is the one that attenuates frequencies higher than a desired frequency and passes the lower frequencies. So How does this definition fit in this case?

The important thing you haven't grasped is that the on-off square wave has an average level that is totally dependent on the duty cycle. This mathematically average level gets converted to a real dc level by the L and C low pass filter action.

Here's a square wave that rises to 3V from 0V (gold trace): -

The blue trace has a little bit of low pass filtering applied - can you see that if a lot more low pass filtering were applied, the blue trace would be almost a constant DC level of 1.5 volts?

Make sure you have ferrite beads that can filter out the higher harmonics of the 2nd buck switching frequency.

^{simulate this circuit – Schematic created using CircuitLab}

This ferrite is shown by L1. Suppose that U2 switches at 200kHz, the MOSFET/switch, keeps switching at that frequency. There will be an impact on the Vin (P1) of the 2nd buck. That noise might percolate ionto the input rail of the 2nd buck or rather the output rail of the 1st buck. As such you might not have much issues. But are you providing the output of the 1st buck to other loads (sensitive) ? if so,especially in case of RF load, this noise can be a killer.

Filter out this with proper ferrites and capacitor banks.