$meta_query = array(
'relation' => 'AND',
'date_clause' => array(
'key' => 'date_field_name',
'compare' => 'EXISTS'
),,
'time_clause' => array(
'key' => 'time_field_name',
'compare' => 'EXISTS'
),
);
$query->set('meta_query', $meta_query);
$orderby = array(
'date_clause' => 'ASC',
'time_clause' => 'ASC'
);
$query->set('orderby', $orderby);