Copy the files:
itemusa.php and itemworld.php
In the proper files:
Globally replace item with itemusa and itemworld
Globally replace ITEM with ITEMUSA and ITEMWORLD
Those are case sensative so be careful ...
WARNING: do NOT change the word item on this line:
Uninstall the old Flat and Per Item that you have installed ...
$total_count = $total_count - $_SESSION['cart']->free_shipping_items();
Install the new ones ... and set their zones
Cloning a shipping module is not difficult just be careful ...