For the time being you can use either approach:

1) Disable ConvertToXhtml filter:

C#:

    protected void Page_Load(object sender, EventArgs e)
    {
        RadEditor1.DisableFilter(EditorFilters.ConvertToXhtml);
    }
ASPX:

        <telerik:RadEditor ID="RadEditor1" runat="server">
            <Content>
            <p>  <h2></h2>  <h2></h2>  </p>
            </Content>
        </telerik:RadEditor>
        <script type="text/javascript">
            Telerik.Web.UI.Editor.NestedElementsFix.prototype._shouldRemoveBlockElement = function (element, elementChanged) {
                var utils = Telerik.Web.UI.Editor.Utils;
                if (!elementChanged || !element) return false;
                if (!/^li|td|th$/.test(element.nodeName) &&
                    utils.isNodeEmptyRecursive(element) &&
                    !$telerik.$(element).find("ol,ul,table,h1,h2,h3,h4,h5,h6").length) {
                    return true;
                }
                return false;
            }
        </script>