The second wire acts to put downward pressure on the cornerpost to negate the upward pull of the original wire.  If I knew how to draw on here I could show you the math of how it works.  Remember trianglulation and all, just instead of lengths, use the forces pulling on the wire and transferred to the post.  Also the wire x tieing pulls the horizontal brace post tight and makes the whole corner move as one instead of individual posts.

When tired of all this, just build pipe corners and set in a foot of concrete and fill remainder of hole with dirt.  Pretty well holds up to anything.