Revolution Slider Orderby Two Custom Fields

To order by event-end date or start date, your write separate function and call them as per requirement. For example if user clicks on order by start then start function should be called.

switch($orderby){
case: 'event-start':
# your code
/*
 $query['meta_key'] = 'event_start_date';
        $query['orderby'] = 'event_start_date';
*/
break;
case 'event-end':
#your code
/*
 $query['meta_key'] = 'event_end_date';
        $query['orderby'] = 'event_end_date';
*/
break;

}