Why are there typically TWO peanuts in a pod, in most cultivars of the peanut?

The fruit is technically called a legume or pod. It is composed of a single seed-bearing carpel that splits open along two seams. Although normally 2 Peanuts per pod they can come in 1, 2, 3 or even 4 in a pod, but that is rare, 2 nuts per pod is more normal.

http://www.okpeanutcomm.org/growing-peanuts-in-the-home-garden.html

Also it depends on which variety, some having more.

I've raised peanuts and know there are many more 3-peanut and 4-peanut pods than would be indicated by the peanuts you buy in the store, which have been culled and graded for uniformity.

Same with odd-shaped potatoes and carrots--they don't all come out of the ground with their characteristic lumpy and conical/cylindrical shape. Potatoes can look like hearts, swans, ducks, hippopotamuses, etc., and carrots like corkscrews or banyan trees.

The variety pf peanut planted makes a difference, too:

"Virginia and Runner types, mostly low-growing plants with usually two large seeds per pod, are the best producers for most gardens; Spanish types are mostly erect plants, and produce pods often having three or four small seeds per pod."

http://www.walterreeves.com/food-gardening/peanut/