I really miss the option “set as start page” in Visual Studio Explorer for Silverlight Projects. If I have time in the next days (;-))Ii will write visual studio extension for that purpose.
In fact it is not complicated. The app.xaml have a codebehind file ( app.xaml.vb or app.xaml.cs) which create a instance of the visible XAML. Btw that is the reason for changing xaml to usercontrol in B2.
All you have to to, to change the start page is to change one line of code in startup. Instead of page write the name of the class you want to show.
Private Sub Application_Startup(ByVal o As Object, ByVal e As StartupEventArgs) Handles Me.Startup
Me.RootVisual = New secondXAML()
End Sub