Trying to perform complex custom field query with order by set to field value

If the value of your custom fields is numeric, you could try ordering with meta_value_num. Also note that both meta_value and meta_value_num require meta_key in the query as described here.

$args = array(
  'post_type' => 'post',
  'posts_per_page' => $number,          
  'meta_query' => array(
    'relation' => 'OR',
    array(
      'meta_key' => 'event-start',
      'value' => $today_is,
      'type' => 'NUMERIC',
      'compare' => '>='
    ),
    array(
      'meta_key' => 'event-end',
      'value' => $today_is,
      'type' => 'NUMERIC',
      'compare' => '>='
    )
  ),
  'orderby' => 'meta_value_num',
  'order' => 'ASC',
);