If you want to globally disable wall clean-up go to View>On-Screen View Options>Clean wall & Beam Intersections and deselect this option. Now all the walls you have drawn will be displayed as individual entities.
If you only want this display on a single corner, put each of the two walls on a separate layer and give each layer a differing intersection number so that they do not clean-up to each other.
Search your help file for "Use Layers to Prevent Wall/Column/Beam Intersections".
