title: Add custom tags for custom post type in WordPress
body: <p>I have many custom post type and I want two of them to be relate by tags. These tags are only useable to these two custom post type only.</p>
  
  <p>For example; I have two custom post type</p>
  
  <ol>
  <li>books</li>
  <li>writer</li>
  </ol>
  
  <p>And I want these two post type have custom tag which only usable to these two post type.</p>
  
  <p>How can I do that?</p>
  
  <p>I try with Custom Taxonomies but didn't success.</p>