Tried like this??
FlexTable f1 = new FlexTable();
f1.setWidget(...
f1.setWidget(...
FlexTable f2 = new FlexTable();
f2.setWidget(...
f2.setWidget(...
FlowPanel flowpanel= new FlowPanel();
flowpanel.add(f1);
flowpanel.add(f2);
RootLayoutPanel.get().add(flowpanel);//may be you forgot this line