There are many ways to do that.
1.- You can create a navigation Page and your L1.xaml and L2.xaml they need to be a Page.
Here is a good article showing this.
http://paulstovell.com/blog/wpf-navigation
2.- You can add in your MainPage a Frame control,with this control you can navigate between pages( again your L1 and L2 must be Pages.( it's almost the same of the first point but the difference you can use the Frame control inside of other control like UserControl.
3.- You can use a ContentControl or ContentPresenter as your container these controls have the property Content when the user click some button to show L1 or L2 xaml (doesn't matter if they are an UserControl or Page) you just need to create a new instance of your view, for example:
MyContentControl.Content = new MyView1();
obviously this technique is very simple and you will need to check what of these options are enough for your requirements.
Best Regards!