The magnifying control can't reuse a layer coming from the map (a layer can only be displayed in ONE control). Instead, create a new layer. Something like:
ArcGISTiledMapServiceLayer arcgisLayer = Map.Layers["imagery"] as ArcGISTiledMapServiceLayer;
ArcGISTiledMapServiceLayer arcgisMagnifyingLayer = new ArcGISTiledMapServiceLayer();
arcgisMagnifyingLayer.Url = arcgisLayer.Url;
MyMagnifyingGlass.Layer = arcgisMagnifyingLayer;
Or even simpler (depending on your need), you could just change the magnifying layer url:MyMagnifyingGlass.Layer.Url = arcgisLayer.Url;