Why 2 kidneys

First, let's review how many of our organs has 2.
External:
Eyes, Ears, Hands, Legs,
Internal:
Lungs, Kidneys, testicles, ovariesPaired external organs of the body is to operate in 3-dimension / directional movements.The reason of two sets of internal organ is not limited by survival, but to handle the possible situation under normal living in minimal function to very extreme conditions. Sometimes, basic survival function may be an extreme situation.To illustrate: If our lung capacity is only limited by one set of air sag just for usual breathing only, then during physical exercise or needed to escape danger in accidents; there won’t be any “spare” air-exchange capacity to generate the extra energy/power. The internal space of our body has limited the volume of air-exchange, to ensure the most extreme situation, two lungs are installed to increase the “spare” capacity by at least 30-40%. [The heart works in double-time: beat twice faster during exercise/dander to effect.]A kidney is a filtration system to remove liquid waste. It does not control the liquid intake; your “mouth” (mind) controls the intake. Since your mind (may decide to drink more when thirsty) or the mouth (may feel thirsty even the stomach is full) or the stomach (which can expand and absorb twice the normal intake) may take in more even during normal living, such as after exercise, at hot/dry area/season, or drink different kinds of liquid that require addition filtration process. To prepare for the worst possible case during normal living, 50% more filters are installed so that the body will not keep the “waste” internally before turns to poison. It is curious that after two kidneys “duplicate” filtration processed the liquid, we only has one bladder to hold/store the waste temporary. It is so such that the body has a built-in “alarm” to reject the waste liquid before the filtration is backed up (blocked).