Have you looked at this SDK sample: http://help.arcgis.com/en/webapi/silverlight/samples/start.htm#MagnifyingGlassMagnifyingGlass need a different instance of TiledMapServiceLayer. Layers already associated to another map will throw an exception.
<esri:MagnifyingGlass x:Name="MyMagnifyingGlass" Visibility="Visible" ZoomFactor="3"
Map="{Binding ElementName=MyMap}" >
<esri:MagnifyingGlass.Layer>
<bing:TileLayer ID="BingLayer" LayerStyle="AerialWithLabels" ServerType="Production"
Token="{StaticResource BingToken}"/>
</esri:MagnifyingGlass.Layer>
</esri:MagnifyingGlass>
or in code-behind:
MyMagnifyingGlass.Layer = new ESRI.ArcGIS.Client.Bing.TileLayer()
{
ID = "BingLayer",
LayerStyle = ESRI.ArcGIS.Client.Bing.TileLayer.LayerType.AerialWithLabels,
ServerType = ESRI.ArcGIS.Client.Bing.ServerType.Production,
Token = this.LayoutRoot.Resources["BingToken"] as string
};