I have a search service application that has 2 content sources - one for the SharePoint Sites and one for File Shares. When I perform a search from the Search Center the results can show documents from both Sites and Shares. How can I show only the results from the Sites or only the results from the Shares?
You need to configure separate Result Sources and add them to the search web parts.
This blog post shows how to create a new Result Source except that on the Build Your Query step you will filter your query to the Content Source as opposed to the ContentClass or Path that are used on the post.
You can then create as many search result pages as you want, filtering for your custom Result Sources as per this second post.