As to why we have two of some things, for the immediate pathological “why” (rather than the evolutionary why),
It seems that the FOXD1 gene “promotes nephron progenitor differentiation”, which I read as meaning it causes a single proto-kidney tissue to split into two distinguishable proto-kidneys. So whatever causes us to have two kidneys, may be contained in that protein.
Here is a study from 2005 where, in their own words,
Genetic deletion of the forkhead transcription factor, Foxd1, results in striking renal abnormalities, including the loss of these discrete zones and pelvic fused kidneys.
Levinson RS, Batourina E, Choi C, Vorontchikhina M, Kitajewski J, Mendelsohn CL. Foxd1-dependent signals control cellularity in the renal capsule, a structure required for normal renal development. Development. 2005 Feb;132(3):529-39. doi: 10.1242/dev.01604. Epub 2005 Jan 5. PMID: 15634693.
~Max