5V USB and 110VAC proper procedure?
Powering your device directly from a 3.3V wall adapter is not the most optimal choice - It's better to power from >5V, then use a POL regulator to get 3.3V. BUt I digress...

Usually it's just fine to parallel two 3.3V rails. There would be no significant effect, especially given the fact that you only use USB occasionally together with Wall. So the 5V from USB would go through your regulator and merge with the AC adapter output. That's totally fine in most cases. On the USB side there is nothing so no worries about current backing or domain issues as well.

You see, if you had used 5V you also would have had more maneuver space for a diode or two.

Optionally, you could implement a power prioritizer or powerpath controller to handle switching.