In the first half of the book, Anne watches Wentworth embark (apparently) on a relationship with another woman. In the second half of the book, Wentworth watches Anne embark (apparently) on a relationship with another man. And in the precise middle of the book, as you point out, is Lyme, the hinge between these two mirror-image stories, and the place where the incident (Louisa's accident) occurs that causes the two protagonists to switch places.

Benwick and Louisa are a precise parallel of Wentworth and Anne in a lot of ways. He's a naval man; she's a 19-year-old daughter of a country gentleman, just as Anne was when her relationship with Wentworth began. Benwick and Louisa are each, briefly, the other half of an unrealized couple with Anne or Wentworth, before they pair off with each other and open the way for the two protagonists to come together. This happens, of course, in other JA novels (think Emma) -- I often think of it as something like a dance, in which the main couple separate, pair off with others and then reunite.