the query arguments would look something like this
$args = array(
'posts_per_page' => -1,
'post_type' => 'event',
'meta_query' => array(
'date_clause' => array(
'key' => 'start_date',
'compare' => 'EXISTS'
),
'feature_clause' => array(
'key' => 'featured', // or whatever your t/f field name is
'compare' => 'EXISTS'
)
),
'orderby' => array(
'feature_clause' => 'DESC',
'date_clause' => 'ASC'
)
);