Does one or the other have prominence?
This is not a real area of expertise for me, Oshburg, but here is one example that comes to mind:
Imagine a newspaper site: On the main page you might do a search on a particular topic and be taken to a search results page. On that page will be a list of articles on your chosen subject (search results, in other words) and also ads. These ads will be in the search network.
Then imagine clicking on one of the search results. You'll then be taken to an article on your chosen subject. On that page will be ads from the content network.
Hope that helps.
AWA