Muirfield and Portmarnock are regarded as the "birthplace" of the two loop routing, at least in their form once the hand of man was allowed in to "fix" things from any accidental routing previously imposed by nature or otherwise.
Also, it must be credited to The Old Course, which at the time of courses being purposefully routed, had by that time the accepted "18 hole" duration. As the Golden Age gained momentum it became fashionable to allow a break within the round, and 1/2 of 18 is, well, go figure.
Mankind strives for organization and balance. It is what we see in patals of flowers, the fingers on one hand versus the other, and in the balance of our own faces. The "half-and-half" approach is actually quite natural, despite its contrived appearance.