const StackA = createStackNavigator({ ScreenA },
{
  defaultNavigationOptions: (navigationOptions),
  navigationOptions: {
    tabBarLabel: 'Screen A',
  },
});

const StackB = createStackNavigator({ScreenA},
{
  defaultNavigationOptions: (navigationOptions),
  navigationOptions: {
    tabBarLabel: 'Screen B',
  },
});

const tabNavigator = createBottomTabNavigator({StackA, StackB});