you can replace TopWindow with ParentCtrl,

class MainWindow : public WithMainWindowLayout<ParentCtrl>, public CallbackQueue, public Reader::Notify, public Writer::Notify, public RpcCmder::Notify

then

	TabDlg dlg;
	MainWindow tab[5];
	
	for(int i = 0; i < 5; i++)
	    dlg(tab[i], String("Tab")<<i);

        dlg .OKCancel()
	   .Title("Tab dialog");
	dlg.Run();