Force new posts to the top of query loop

Here’s a suggestion using posts_orderby filter and CASE, UNIX_TIMESTAMP and DATEDIFF (that outputs number of days) from MySQL for conditional SQL ordering:

add_filter( 'posts_orderby', function( $orderby, $q ) use (&$wpdb) {
    if( ! $q->is_main_query() ) { 
        return $orderby;
    }
    return $wpdb->prepare(
        'CASE 
            WHEN DATEDIFF( %i.post_date, NOW() ) <= 14
                THEN -UNIX_TIMESTAMP( %i.post_date )
            ELSE %i.post_title
         END ASC',
        "{$wpdb->posts}",
        "{$wpdb->posts}",
        "{$wpdb->posts}"
    );    
}, 10, 2 );

This might probably be simplified with e.g. numbered arguments like %1$i if that’s supported by the new prepare-placeholder %i for table/field names.

To avoid using two order-by fields with two cases, to handle both ASC and DESC ordering, I instead suggest using a single order-by field with a single ASC order by using negative unix timestamps for the date ordering.

Hope you can test this further.