Tapped buck converter
You can sortof do it, but it's probably not worth the trouble.  You would have to add a diode in front of the 5V output cap, and a pass element in front of the 3.3V output cap.  The switcher regulates the 5V output, and a comparator turns on the pass element whenever the 3.3V output is low.  In other words, the comparator via the pass element steals current that would otherwise go to the 5V supply, but the feedback system makes sure the 5V supply gets enough current to keep it at the right voltage.  Like I said, probably not worth it.

To do this in one switcher somehow will require a inductor that can deliver 3A average.  It won't be much worse to split that into two inductors that each only have to deliver 1.5V.

Another possibility is to have the switcher make 5V, then linearly regulate that to 3.3V.  This wastes 2.6W in heat.

You could buck to make the 5V, then use a open loop fixed duty cycle buck converter with 3.3V/5.5V = 66% duty cycle to make the 3.3V from there.