Invoegen in 2 tabellen met PDO MySQL

Dat is geen geldige SQL; je moet ze apart doen. Misschien:

$sql = "
    INSERT INTO `directory`(`First_Name`,`Surname`,`Nicknames`) VALUES (:firstname, :surname, :nicknames);
    INSERT INTO `nicknames`(`First_Name`,`Surname`,`Nicknames`) VALUES (:firstname, :surname, :nicknames);
";