<script type="text/javascript">
        function test() {
            var t1 = document.getElementById("editContent");
            var t2 = document.getElementById("editDiscription");
            t2.value = t1.value;
        }
    </script>
<form id="form1" runat="server">
    <div>
        <asp:TextBox ID="editContent" runat="server"></asp:TextBox>
        <asp:TextBox ID="editDiscription" runat="server"></asp:TextBox>
    </div>
    </form>

 protected void Page_Load(object sender, EventArgs e)
    {
        editContent.Attributes.Add("onkeyup","test()");
        editDiscription.Attributes.Add("onblur","test()");
    }