I'm currently studying different approaches to supplying a PCB which needs both 3.3V and 5V from a wide range DC input source (i.e., 6-36V.)
The most common layout I've come across is V_in(6-36) -> buck (3.3) -> boost (5.) Is there a reason for this being the most common? What are the potential drawbacks of V_in (6-36) -> buck(5) -> buck(3.3,) or not putting them in series at all and simply using two buck converters side by side?
If the 5V requires only a small current, using a boost (or even a flying-cap converter) from 3.3V would give wide flexibility for the input voltage. For higher 5V current it could instead be a buck-boost directly from Vin, which would be more efficient than boosting from the 3.3V rail.
One challenge you face is the large input voltage range required. This has a profound effect on the DC-DC: the stepping ratio down to 3.3V varies from about 1.8:1 (55% duty cycle) for 6V to about 11:1 (9% duty cycle) for 36V. Why does this matter? It affects the inductor choice and switching frequency: the higher the stepping ratio, the shorter the minimum on-time becomes.
If you can step down to 5V instead for your bulk voltage, then make 3.3V from that, then these extents become 83% and 14% for 6V and 36V, respectively. This will allow a higher switching frequency. As it is, many DCDC's will run at close to 100% duty, so the lower input is doable even though the Vin-Vout overhead voltage is just 1V (some regulators will even switch to linear mode as Vin gets close to Vout.) Meanwhile your 3.3V gets to be a low-voltage input type, so possibly less expensive and able to run at a higher switching frequency.
Because the 3.3 V current is so small, I would use a buck regulator to take the input down to 5 V, and a linear regulator to pick off some of the 5 V and turn it into 3.3 V. 0.7 W is not a lot of heat to move.
