Double Select - n:n (many-to-many)

How can I configure the double select n:n many to many to display the records on the right side every time I change the value of the combo box. Because the example in the website “http://www.scriptcase.net/scriptcase-samples/php-form/update-a-link-table/” every time I change the region it display empty record on the right side of the double select it should display which I choose or which I update.

Double Select.jpg