Overlapping Combo boxes

Why do you want them to overlap? You might want to consider repopulating a single combobox with a different list of items instead. See this article:

[url=http://www.acrobatusers.com/tutorials/2007/js_list_combo_livecycle/]http://www.acrobatusers.com/tutorials/2007/js_list_combo_livecycle/[/url]

But, you can always write code to hide the unwanted form fields. The problem is figureing out which ones to hide. What do you mean by selected? Are there radio buttons next to comboboxes? If you have a way to figure out which one is "selected", then you can attach code to the flatten script for hiding the unwanted fields.