title: How to order by multiple meta keys?
body: <p>I’m using a custom loop to display my events on a page, I get it fine ordering by the event start date using the below:</p>
  
  <pre><code>$args = array( 
      'post_type' =&gt; 'event',
      'order' =&gt; 'ASC',
      'orderby' =&gt; 'meta_value',
      'meta_key' =&gt; '_event_start_date');
  $loop = new WP_Query( $args );
  </code></pre>
  
  <p>But the meta_key option only allows one value. How to use two values (<code>_event_start_date</code> and <code>_event_start_time</code>)? </p>