2 Confirmational messages when clicking on button in asp.net

Please refer
http://www.aspsnippets.com/Articles/Calling-server-side-function-from-JavaScript-in-ASP.Net.aspx[^]

You can use JavaScript confirm box
You need to add confirm box on OnClientClick event of button .when a user response to first confirm box then catch the result of first confirm box .pop up second confirm according to response .
save second confirm box values in hidden field and use that to execute your C# function in code behind

suppose you have one hiddenfield name hdnconfirm

In your aspx file

<asp:Button ID="btnInvoke" runat="server" Text="Click"
                    onclick="btnInvoke_Click" OnClientClick="return confirmation();" />
<asp:HiddenField ID = "hdnconfirm" runat="server" Visible="true" />

in head section of your asp page

<script type="text/javascript" language="javascript">
       function confirmation() {
           var FirstConfirmResult = confirm("Click yes to proceed else Cancel");
           if (FirstConfirmResult == true) 
           {
             var SecondConfirmResult =   confirm("This is second confirm box ");
            
              if (SecondConfirmResult == true )
               {
               

                    document.getElementById('hdnconfirm').value = "Yes";
                
               }
              else 
              {
                document.getElementById('hdnconfirm').value = "No";
              }
            
           }
       }


   </script>

Use this hdnconfirm.value in code behind to execute your function

protected void btnInvoke_Click(object sender, EventArgs e)
   {
     

       if (hdnconfirm.value == "Yes")
       {
          
              Do some processing here
       }
   }

Hope This is what you need !