<%@ Control Language="c#"  Inherits="FSB.Layouts.Short_Home_Loan_Application" CodeBehind="Short Home Loan Application.ascx.cs" %>
<div class="form">
<form action="" method="post" rel="shortForm">
    <div class="section">
         <asp:panel id="shortForm_wrapper" runat="server">
            this is the first panel
         </asp:panel>
         <asp:Panel runat="server" ID="thankYouWrapper" >this is the second panel</asp:Panel>
    </div>
</form>
</div>

<sc:sublayout runat="server" path="~/layouts/FSB/Document Checklist Lightbox.ascx" id="checklist" />

using System;
using Sitecore.Data.Items;
using Sitecore.Links;
using Sitecore.Data.Fields;
using Sitecore.Web.UI.WebControls;
using System.Net;
using FSB.Helpers;
using System.Configuration;
using System.Text;
using System.Web;
using System.Web.SessionState;

namespace FSB.Layouts {

    public partial class Short_Home_Loan_Application : System.Web.UI.UserControl {

        protected void Page_Load(object sender, EventArgs e) {
            thankYouWrapper // not accessible here
        }
    }
}

<%@ Control Language="C#" Inherits="FSB.Layouts.Short_Home_Loan_Application" CodeFile="Short Home Loan Application.ascx.cs" %>

CodeFile="Short_Home_Loan_Application.ascx.cs" %> //AddCodeFile, and set the same name