I'm constructing a WebMap through ArcGISOnline Web Mapping Application. I've been able to add both a colored canvas basemap and a satellite imagery basemap to the map, but only with one covering the other. Is there an option to add a toggle or switch on the final webmap to let the viewer switch between the basemap layers?
You said you have built a web map in ArcGIS Online? The next step would be to create a web map application in Web App Builder and choose the web map as the map for the application. Then you will configure the basemap widget to use the two basemaps you have chosen. It is not necessary to have both the basemaps in your web map for this. Below is a link on how to accomplish this: