In a page contains two `UpdatePanels`, How can I know which `UpdatePanel` causes the partial `PostBack` ?

I mean in the `Page_Load` event handler.

This is my code:

```
 <asp:ScriptManager ID="ScriptManager1" runat="server">
 </asp:ScriptManager>
 <asp:UpdatePanel ID="UpdatePanel1" runat="server" UpdateMode="Conditional" 
     onprerender="UpdatePanel1_PreRender">
     <ContentTemplate>
         <A:u1 ID="u1" runat="server" />
     </ContentTemplate>
 </asp:UpdatePanel>
 <asp:UpdatePanel ID="UpdatePanel2" runat="server" UpdateMode="Conditional" 
     onprerender="UpdatePanel2_PreRender">
     <ContentTemplate>
         <A:u2 ID="u2" runat="server" />
     </ContentTemplate>
 </asp:UpdatePanel>

```

I tried this code, but it isn't worked alost!

```
protected void Page_Load(object sender, EventArgs e)
{
    if (ScriptManager.GetCurrent(Page).IsInAsyncPostBack)
    {
        if (UpdatePanel1.IsInPartialRendering)
        {
            // never enter to here
        }
        if (UpdatePanel2.IsInPartialRendering)
        {
            // neither here
        }
    }
}

```

Any help!