If you use some db functions, it is possible, heres how i once did it after the userstats insert add this:
$change = $db->change_db('forms');//Forums db name
if($change === FALSE) { echo mysql_error(); }
$db->query("INSERT INTO forum_members (is_activated, memberName, passwd, emailAddress, gender, dateRegistered, realName)
VALUES ('1', '{$username}', sha1('{$_POST['password']}'), '{$_POST['email']}', '{$_POST['gender']}', unix_timestamp(),'{$username}' )"); //Insert into forums from the post above
$changeback = $db->change_db('game');//Game db name
if($changeback === FALSE) { echo mysql_error(); }