On the Developers web page at:
Build a new map—ArcGIS Runtime SDK for .NET | ArcGIS for Developers
is the following topic:
Combining layers into one basemap
You can also combine a number of layers to create a single basemap as long as these layers have the same spatial reference
However, the two ArcGIS Online layers included in the sample code under that topic appear to have different Spatial Reference WKId's. So, what constitutes "same spatial reference"?