This article will get you the first drop down:
http://www.tmib.net/agencies-dropdown-and-admin-ui-osticket-17st(http://www.tmib.net/agencies-dropdown-and-admin-ui-osticket-17st)
Once you have that in place and working then you need to create the second one almost the same way. However you would use an onselect or onchage and some javascript as glue to call another php file to populate the 2nd drop down. This type of thing is usually called AJAX.
This article may assist you further:
http://www.sitepoint.com/forums/showthread.php?442811-How-can-an-onChange-event-%28of-a-dropdown%29-call-php(http://www.sitepoint.com/forums/showthread.php?442811-How-can-an-onChange-event-%28of-a-dropdown%29-call-php)