List Select Transfer

What i would like to be able to do is have two listboxes side by side, one will have all of the main categories and the second will have all of the sub categories in it.
I populate the first list by using this code:

<option value= "<% = rs("cat_id") %>"><% = rs("cat_name") %></option>

Now what i would like to do is when the user double clicks say the second item on the list, the listbox next to it will display all of the items that are under the main category they just double clicked on