Hi Hedge,
To be clear, you may only extend a wall once with the same cast action. i.e. You may cast 1 or 2 walls.
From the rulebook:
"The second wall must be placed as a continuation of the first wall, either in a straight line or bending around a corner. Both walls must be in range and LoS as normal."
I drew a picture to help visualize the situation:
[attachment=39]WallPlacement.png[/attachment]
Red Line: The first wall that was cast.
Green Lines: Potential placements for the second wall to be cast.
M = Casting mage.
Hope this helps!