<Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="Auto" />
        </Grid.RowDefinitions>
        <Button
            Grid.Row="0"
            Height="40"
            Command="{Binding ClickCommand}"
            Content="Click" />
        <ucerControls:FirstUC Grid.Row="1" DataContext="{Binding FirstViewModel}" />
        <ucerControls:SecondUC
            Grid.Row="2"
            DataContext="{Binding SecondViewModel}"
            Visibility="{Binding Visible}" />
        <Button
            Grid.Row="3"
            Height="30"
            Content="{Binding Text}"
            Visibility="{Binding Visible}" />
    </Grid>

public class MainViewModel:INotifyPropertyChanged
    {

        #region PropertyChangedEventHandler
        /// <summary>
        /// Метод проверяющий изменилось ли свойство
        /// </summary>
        public event PropertyChangedEventHandler PropertyChanged;
        protected virtual void OnPropertyChanged(string propertyName)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
        #endregion

        private FirstViewModel _firstViewModel;
        private SecondViewModel _secondViewModel;
        private string _visible;

        public MainViewModel()
        {
            FirstViewModel=new FirstViewModel();
            SecondViewModel=new SecondViewModel();
            Visible = "Collapsed";

            ClickCommand = new Command(arg =>
            {
                Visible =(Visible=="Collapsed")?"Visible":"Collapsed";
            });
        }

        public FirstViewModel FirstViewModel
        {
            get { return _firstViewModel; }
            set { _firstViewModel = value; OnPropertyChanged("FirstViewModel");}
        }

        public SecondViewModel SecondViewModel
        {
            get { return _secondViewModel; }
            set { _secondViewModel = value; OnPropertyChanged("SecondViewModel");}
        }

        public string Visible
        {
            get { return _visible; }
            set { _visible = value; OnPropertyChanged("Visible"); }
        }

        public ICommand ClickCommand { get; set; }
    }