If I had the draconic bloodline, and gained:
Dragon Resistances (Ex): At 3rd level, you gain resist 5 against your energy type and a +1 natural armor bonus. At 9th level, your energy resistance increases to 10 and natural armor bonus increases to +2. At 15th level, your natural armor bonus increases to +4.
And I had an Amulet of Natural armor:
This amulet...toughens the wearer’s body and flesh, giving him an enhancement bonus to his natural armor from +1 to +5, depending on the kind of amulet.
...would the two natural armor bonuses stack?
yes, becuase one is natural armor bonus while the other is an enhancement bonus to that natural armor.
this is the same reason a '+1 chain shirt' give a total of +5 to ac. +4 armor bonus and +1 enhancement bonus to that armor.
but the same kind won't stack:
- so if you have racial +1 to nat ac (say you are of a Nagaji race) that won't stack with the blood line (unless it specifically call out to do stack).
- and the barkskin spell which give enhancement bonus to nat ac won't stack with the amulet.