I solve the problem like this (although this is not exactly what your asking for)

I add this to the top of the skin (ascx skinning):

<script runat="server">

    Private Function ParentId(optional iLevel as integer = 0) as integer
    'Return the Id of the active parent Tab in the current Breadcrumb on the passed level, defaults to the root tab
            try
            Dim objTab As DotNetNuke.Entities.Tabs.TabInfo = CType(PortalSettings.ActiveTab.BreadCrumbs(iLevel), DotNetNuke.Entities.Tabs.TabInfo)
            return objTab.TabId
            catch
            end Try
    End Function

</script>

Then for the second menu I use this:

<DNN:HOUSEMENU1 id="dnnHOUSEMENU1" Runat="Server" IsRecursive="True" ShowPageIcons="False" Scope="<%#ParentId%>" ShowAdmin="True" ShowParentTab="false"></DNN:HOUSEMENU1>

It will render an empty div if the parent page has no children.