ORing 2 power supplies for 3.3V output
If you don't care about efficiency, use two diodes, like below:







simulate this circuit &ndash; Schematic created using CircuitLab

Otherwise use:


Ideal/smart bypass diodes (such as SM74611) instead of D1 and D2.
Some ORing FET controller such as LM5050 + nMOSFETs.
Some some autoswiching power mux.


If, instead, you must have 2 separate 3.3V regulators, then use something like a TPS2113 (2.8 to 5V autoswitching power mux).