You have 2 Pages and you want to define which one will automatically load when the App starts?
If so, in solution explorer, go to, Properties > WMAppManifest.xml.
If you are in designer mode, go to Application and edit the "Navigation page", to the page you want to load when the App starts.
Or, if you have code view, look for the "Task" Node and change the "NavigationPage" attribute of the default task.
<Tasks>
<DefaultTask Name="_default" NavigationPage="Page1.xaml" />
</Tasks>