Think of it like a spline object. You can't have a true "T" joint, just two segments can be joined and a third butting up against it. So in your example, it would look better if the left and bottom walls were connected at the joint, and the bisecting wall were butting up to the inner angle. It won't be perfect, though.
AEC Walls are not all that great, and if you can, I would suggest extruding splines to make those walls. Although, you won't be able to add doors and windows in the same way. Alternatively, after creating the AEC walls, add an Edit Poly (or Mesh) to clean up the corner.
Chris Medeck
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