What is the evolutionary advantage with two lungs (kidneys)?
Humans and most organisms have bilateral symmetry.  Anything that is on the left side of the body will be on the right also.  Not everything can be in the middle, so ... you end up with two.  This is evolutionarily labile - for example, fish generally have one dorsal swim bladder, though it is derived from the lungs.  In a similar manner, the heart is formed by the fusion of primordia on either side during development - see UNSW for a cute video.