Possible to merge these two functions?

Unless I’m missing something, I would think you could just use an OR (||) operator to check if it is_tag() or is_tax():

add_action( 'pre_get_posts', 'sort_conference_by_date' );
function sort_conference_by_date( $query ) {
    if ( $query->is_main_query() && !is_admin() ) {
       if ( $query->is_tag() || $query->is_tax() ) {
            $query->set('orderby', 'meta_value');  
            $query->set('meta_key', 'start_date');  
            $query->set('order', 'DESC'); 
       }       
    }
}