Код:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
 
namespace WpfApplication2
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            ta.Completed += ComplitedAnimation;
 
            ta.FillBehavior = FillBehavior.Stop;
            ta.Duration = TimeSpan.FromSeconds(0.5);
            ta.Duration = new Duration(new TimeSpan(0, 0, 5));
        }
 
        ThicknessAnimation ta = new ThicknessAnimation();
 
        public double main_window_width { get { return main_window_grid.ActualWidth; } }
        public bool fl1;
        public bool fl2;
        private Thickness marginBackup;
 
        private void ComplitedAnimation(object sender, EventArgs e)
        {           
            if (fl1 == true)
            {
                fl1 = false;
                grid_blue.Visibility = Visibility.Visible;
                grid_red.Margin = marginBackup;
            }
 
            if (fl2 == true)
            {
                fl2 = false;
                grid_red.Visibility = Visibility.Collapsed;
                grid_blue.Margin = marginBackup;
            }
 
        }
 
        private void Move_Animation(object sender, RoutedEventArgs e)
        {
            fl1 = true;
            ta.From = new Thickness(main_window_width, 0, 0, 0);
            ta.To =  grid_red.Margin;
            marginBackup =  grid_red.Margin;
          
            grid_red.BeginAnimation(MarginProperty, ta);
            grid_red.Visibility = Visibility.Visible;
 
            ta.From = grid_blue.Margin;
            ta.To = new Thickness(0, 0, main_window_width, 0);
            grid_blue.BeginAnimation(MarginProperty, ta);
        }
    }
}

Код:

<Window x:Class="WpfApplication2.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid x:Name="main_window_grid">
        <Grid>
            <Grid x:Name="grid_blue"  Height="200" Width="350" Background="Blue" Margin="0,0,0,0">
            <Button Name="bt_blue" Width="50" Height="50" Margin="200,100,0,0" Content="button" Click="Move_Animation"></Button>
        </Grid>
        <Grid x:Name="grid_red"  Height="200" Width="350" Background="red" Margin="0,0,0,0" Visibility="Collapsed">
                <Button Name="bt_red" Width="50" Height="50" Margin="200,100,0,0" Content="button"></Button>
        </Grid>
        </Grid>
    </Grid>
</Window>