Binding One ListBox to Two Dropdown Box Controls

I have a listbox that needs to be bound to two controls.  
There is a button that clears any previously selected data so the listbox will always be empty,
How can I bind the second dropdown list to my listbox?
Here is my code:
asp:DropDownList runat="server" ID="lbAuthorsList" style="float:left;"         DataSourceID="AuthorsList" DataTextField="DisplayName" DataValueField="UserID"         onselectedindexchanged="AuthorsList_SelectedIndexChanged"         AppendDataBoundItems="True"  AutoPostBack="True"         ontextchanged="AuthorsList_SelectedIndexChanged" >          <asp:List ...