Effect of using two buck converters in series
Make sure you have ferrite beads that can filter out the higher harmonics of the 2nd buck switching frequency. 







simulate this circuit &ndash; 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.