title: How to make two backbone fetch calls and combine it
body: <p>I need to call two back end calls on the same collection and merge the results it to single collection object. Also I need to trigger 'sync' method only once both of the fetch methods are success. </p>
  
  <p>Here is the code snippet </p>
  
  <pre><code>myCollection.fetch({ headers: {myheader: 'test1'}, data: {status: 'test1'}, reset: true});
  myCollection.fetch({ headers: {myheader: 'test2'}, data: {status: 'test2'}, reset: true, add: true});
  
  myCollection.once('sync error', function () {
      $('#js-my-grid').html(view.render().el);
  
  }); 
  </code></pre>
  
  <p>I need to render the view only once both of the fetch are completed. How is it possible ? </p>
  
  <p>Thanks in advance.</p>