If I properly understand you need to place 2 label one over another.
For this you can try:
FlowLayoutContainer somePanel = new FlowLayoutContainer();
mixCont.add(somePanel);
Label versionLabel = new Label("12345678");
Label expirationMessage = new Label("9999999");
versionLabel.getElement().getStyle().setFloat(Style.Float.LEFT);
expirationMessage.getElement().getStyle().setPosition(Position.ABSOLUTE);
expirationMessage.getElement().getStyle().setLeft(20, Unit.PX);
somePanel.add(versionLabel);
somePanel.add(expirationMessage);
Actually next code should solve your problem. Second row helps you in positioning your overlapping message:
expirationMessage.getElement().getStyle().setPosition(Position.ABSOLUTE);
expirationMessage.getElement().getStyle().setLeft(20, Unit.PX);