Viewshed layers can only be added to a map using data from one grid-based layer at a time. If you have two DEM files, you can create a map with two layers in it (e.g. two contour layers, one for each DEM), but the viewshed can only be added to one layer in the map or the other, not both.
In this case, you can use the Grids | Resize | Mosaic command to merge the two DEMs together into one grid file. Then use the combined grid file to create the contour map, and add the viewshed to the single contour layer.
Updated October 16, 2018