Custom Control Referring To External Control?

[lowerbackpain]

I'm creating a control inheriting from ListBox that will paste the selected ListItem's value into another, external, control defined in a property of the ListBox control. Is it possible to get the client ID from the external control in the code of the ListBox control? (Without writing additional code in the aspx containing the control.)

My ASPX would look like this;

<cc:ListBox2 id="lstBox" runat="server" CopyToControl="txtSomewhereElse" SelectionMode="Multiple">
</cc:ListBox2>

<asp:Panel id="pnlSomewhereElse" runat="server">
<asp:TextBox ID="txtSomewhereElse" runat="server" />
</asp:Panel>

I imagine it can be done considering the ASP.NET validation controls (appear to) do it. But how?