Silverlight does not have the Page type, the term is currently used is RootVisual UserControl. RootVisual object is analogous to the root window in WPF and can only be set once for the lifetime of the app, and is effective once the Application’s Startup event is raised. But this not true, you can set RootVisual as many times you want. So Application_Startup can be used to call multiple Silverlight controls.

    <form id="form1" runat="server" style="height:100%;">
        <asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
        <div  style="height:100%;">
            <asp:Silverlight ID="Xaml1" runat="server" InitParameters="ControlId=Page" Source="~/ClientBin/SilverlightApplication1.xap" MinimumVersion="2.0.30523" Width="100%" Height="100%" />
		<object data="data:application/x-silverlight," type="application/x-silverlight-2-b2" width="100%" height="100%">
			<param name="source" value="ClientBin/SilverlightApplication1.xap"/>
			<param name="onerror" value="onSilverlightError" />
			<param name="background" value="white" />
			<param name="initParams" value="ControlId=BookStore" />
			<a href="http://go.microsoft.com/fwlink/?LinkID=115261" style="text-decoration: none;">
     			<img src="http://go.microsoft.com/fwlink/?LinkId=108181" alt="Get Microsoft Silverlight" style="border-style: none"/>

and in the App.xaml.cs

        private void Application_Startup(object sender, StartupEventArgs e)
            if (e.InitParams.ContainsKey("ControlId"))
                switch (e.InitParams["ControlId"])
                    case "BookStore":
                        this.RootVisual = new BookStoreWithConverters();

                    case "Page":
                        this.RootVisual = new Page();