Extruding two sides at 90° angle What would you say is the easiest way of extruding the two faces at the first screen so that the model ends up looking like the second?
In these situations I like to delete the edge and use F2 add-on for reconstructing the geometry(F2 comes with Blender and just needs to be activated in the Preferences). I have it enabled anyway for modeling and this process is quite pleasant because of the simplicity of it - only pressing f a few times: