Well, MDI are not supported in WPF and this is not a hack to create MDI applications in WPF. What we will be doing is to simulate MDI like behavior. The main window has two buttons one for Screen1 and another for Screen2. The XAML is shown below.
The Code Behind for the XAML is.
Screen1 and Screen2 are User Controls whose XAML is shown below.
When you click Screen1 button, screen1 appears and clicking screen2 shows screen2. Note that it is the same User Control that will be showed again and again. So the state should be essentially saved. Much like Page Navigation without crying out loud.