Tom:
Stonewall Two has one of the most interesting stories behind it.
The original course was built with the idea of being out in the country, and to be a bastion for walking and caddies and no golf carts. To the first end, the founders optioned some of the property which overlooked the third hole, and bought it a few years later.
The first course only plays about 11,000 rounds per year, so there was really no need for a second eighteen ... but the founders started to become afraid that the next generation would allow carts on the course as they became older. Essentially, they built the second course to ensure that the original would be preserved as the walking-only enclave which it was intended to be.
(Ironically, the second course is easier to walk than the first, but you can get a cart on a hot sweaty summer day!)