You can add script to the 'change' event in the first drop-down to repopulate the second drop-down with the items you need.
So in DropDownList1's 'change' event you'd add JavaScript code like this:
DropDownList2.clearItems();
DropDownList2.addItem("MyFirstItem");
DropDownList2.addItem("MySecondItem");
Filling in the 2nd list with whatever's appropriate based on what was selected in the first list.
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SteveX
Adobe Systems