UserControlA.xaml 
 <WrapPanel Background="Azure">
        <TextBlock Text="This is UserControlA" Width="200" Height="60"/>
        <TextBox Text="{Binding Info}" TextWrapping="Wrap" Width="300" Height="60"/>
  </WrapPanel>
  
  UserControlB.xaml
   <StackPanel>
        <TextBox x:Name="UBTxt" Text="{Binding InfoFrom,Mode=TwoWay}"></TextBox>
        <Button Name="Btn" Content="This is UB,Add text to UA" Command="{Binding}"  Click="Btn_Click"></Button>
   </StackPanel>
   
   UserControlB.xaml.cs
   public partial class UserControlB : UserControl
    {

        public static string infoB;
        public UserControlB()
        {
            InitializeComponent();
        }

        private void Btn_Click(object sender, RoutedEventArgs e)
        {
            infoB = UBTxt.Text.ToString();
        }
    }
	MianWindow.xaml
	 <Window.DataContext>
        <local:ViewModel></local:ViewModel>
    </Window.DataContext>
    <StackPanel>
        <local:UserControlA></local:UserControlA>
        <local:UserControlB></local:UserControlB>
    </StackPanel>
	
	ViewModel.cs
	public class ViewModel : INotifyPropertyChanged, ICommand
    {
        public string info;
         public string Info
        {
            get
            {
                return info;
            }
            set
            {
                info = value;
                OnPropertyChanged("Info");
            }
        }


        public void Execute(object parameter)
        {
            Info = UserControlB.infoB;
        }

        public bool CanExecute(object parameter)
        {
            return true;
        }

        public event PropertyChangedEventHandler PropertyChanged;
        public event EventHandler CanExecuteChanged;

        private void OnPropertyChanged([CallerMemberName] string propName = "")
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propName));
        }
    }