In this scenario we may have 20 Organizations with 30 Department Addresses. If I put the Org's in one combo box there of course would be 20 selections. If I placed all the Department Addresses in the next combo box there would be 600 selections and nothing assuring that the Department Address chosen was in the correct Org.
If Org 1 is chosen then the next combo box selection will be only those addresses under Org 1. If Org 2 is selected the next combo box selection will be only those Department Addresses under Org 2