In general, the first layer in the map's layers collection will set the map's spatial reference. If the spatial reference of a tiled map service layer does not match the spatial reference of the map, it will not display. Bing Maps tiled map services host map data in the Web Mercator projection which is represented as a spatial reference with a well-known ID (WKID) of 102113. If the Bing Maps layer is first, the USGS Topographic layer (if a tiled map service) must have a spatial reference that matches a WKID for Web Mercator (102113,102100,3857). If it does not, it will not display. The reverse also applies (USGS layer first, Bing second).
