Voltage Regulators Being Used - Arduino Mega 2560 and Arduino Due

So I was wondering, if I put an external 5V to the 5V pin on the mega 2560, would the 3.3V pin still work? I can see only 1 voltage regulator on it...

I'm also curious about the Arduino Due, same question as above but also: It is stated that the DC current is the same for both pins, how is that if I only see one regulator...? Furthermore, if the board will be supplied 3.3V to the 3.3V will the board work or the 5V is actually being used even though the chip operates at 3.3V natively...

I don't have one but the schematic does show a 3.3 reg.

Oh now I see. Looking at the schematic of mega 2560, it has LP2985-33DBVR (low-dropout) for outputing 3.3V. And NCP1117ST50T3G (low-dropout), for outputing 5.0V. I think not very efficient, is it?

As for the due, it has LM2734Y (switching-regulator) for 5.0V. And NCP1117ST33T3G (low-dropout) for 3.3V.