Two input voltages to buck voltage regulator, using diodes to protect the sources
You have the right idea but the wrong implementation.  Yes, the concept of one diode in series with each source is good.

However, 1N4148 is totally inappropriate here.  That's a small signal diode, not a power diode.  Let's say for example that the buck regulator is 85% efficient.  With 1 A at 3.3 V out, it needs 3.88 W in.  When only the 5 V source is active, then the input voltage is about 4.3 V.  That means the input current will be about 900 mA.  That's way too much for a 1N4148.

Since your voltages are low, you should use Schottky diodes.  Schottky diodes that can withstand 30 to 40 V and handle 1 A are cheap and easy to find.  The lower forward voltage drop will also increase overall efficiency a bit, especially from the 5 V source to the output.