Bypassing a switching regulator
In this situation you would typically design an active switching solution.

First you nominate a primary supply - the supply that is the preferred one. Normally this would be the higher powered supply, so your switching regulator.

If that supply is working, then the secondary supply should be isolated. If it's not working, then the secondary supply should be enabled.

The trick here is to use a component backwards - a P-channel MOSFET.  Connect it in series with the 3.3V input in such a way that its parasitic diode conducts.  This provides a lower (~3V) "trigger" voltage.  You then use that to power an op-amp, which works as a comparator.  The input (5-12V) voltage is then monitored through a resistive divider to ensure it is above a suitable threshold by the op-amp.  If the input voltage is below that threshold then the output of the op-amp enables the P-channel MOSFET, which then causes its parasitic diode to be bypassed by the main channel, thus reducing the voltage drop to near 0.

It's basically how boards like the Arduino UNO etc handle multiple power sources and yet get the full 5V from the USB power input:



Your equivalent would have USBVCC as your 3.3V input, and +5V would be your intended 3.3V running voltage.