Certainly not so we can donate them to others!
Is it a myth that we can be fully functional with only one kidney?
If true, what is the evolutionary advantage of having two?
Well, I suppose the advantage to having two is that you can’t live with none. Since evolution didn’t anticipate transplant technology, it came up with its own redundancy plan.
This post bought to you by the Council for Anthropomorphizing Scientific Theories. CAST: Downsizing Mother Nature and Father Time since 1924.
When I was in med tech school, we were told that we could function with one kidney. It would be running at nearly full capacity, but it could keep up. Two gives you a lot more room to handle acute overloads of salt, water or other things that put a large metabolic or functional load on the kidneys.
Vlad/Igor
And I suppose that having redundancy via two seperate organs, instead of just one with extra capacity, allows you protections against site-specific injury to a kidney, and also fits in well with our bilateral symmetry thing.
I suspect that this sort of organ formation is something that fits in pretty well with traditional darwinian genetics… the combinations of genes that determine internal organ structure may be fairly complex, (though not nearly as complex as the ones that play a part in neuron pattern formation,) but whichever combinations lead best to survival, especially survival during youth, will probably get passed along to suceeding generations. It’d be interesting if the two kidneys thing was originally a simple mutation that just managed to catch on at some point.
Would we be better off with two hearts? 
Well, that’s the question, innit? Why don’t we have two of everything, and for those things we have two of, why not three or four? The easiest answer is that there is no answer for “why”, when it comes to evolution, other than, " 'cause that’s what happened to work." By definition, it was the best accidental solution to a problem that impeded reproduction. There’s no plan or design about it.
You do, indeed, have two kidneys in case I need one…and please make sure you save it for me.
One kidney is totally fine for an otherwise healthy person; most systemic diseases tend to have the same effect on both kidneys so having two doesn’t seem like it would confer that great an evolutionary advantage. Injury or isolated non-cancerous masses wrecking one kidney and leading to a survival advantage of having two seems like an infrequent circumstance…
If you look at the way we develop from embryos (and I sorta flunked embryology, so take this w/ a grain of salt) we are two halves that come together. Go back far enough and the presumption is that whatever causes cell differentiation doesn’t happen til there are at least two cells interacting in some way. (And even there, if you separate those two cells completely, you’d get “identical” twins.) When we don’t combine the two halves back together properly, we get stuff like spina bifida and plenty of other bad stuff.
Each half gets its own kidney and there was no survival advantage to making one organ out of it.
FWIW it’s reasonably common to have a single combined kidney, or some other oddball variation of the whole urinogenital tract.
Not too common for a guy to get two of his Main Organ (unfortunately?), although I thought a saw a post somewhere around here referencing such a blessed event.