Hi,
here is an example with screenshots:
http://www.assuredynamics.com/index.php/category/portfolio/linking-dropdowns-using-global-binding/
It doesn't involve scripting, so you just need to look at how the two dropdowns are set up.
the first (ID) has display items set in the Object > Field palette. This is what the user sees when they open the dropdown. In the Object > Binding palette there are specified bound items for each of the display items in the list. These are the descriptions for each of the IDs. When the user select an item in the dropdown, the corresponding bound item is the VALUE of the dropdown.
The description dropdown has display items set. These must be the exact same as the bound items for the ID dropdown.
Lastly both fields have the same name and have their binding set to global, in the Object > Binding palette. This way no matter which dropdown the user selects from, the data will be synchronised.
Hope that helps,
Niall
Assure Dynamics