Is this what you want? What I did is made the Distance Parameter with the Base (look to the Command line for the option) at the Midpoint, and associated two stretch actions with it. One acted on just one side of the wall and had an "Angle Offset" of 0; the other acted on the other side of the wall and had an "Angle Offset" of 180 (which makes it act in the opposite direction. I would start reading >>
here<< and just play around with the properties. The properties palette and the command line are going to be your best friends when making dynamic blocks.
BTW: There are some great people over at the
AutoDesk Dynamic Block forum that really know their stuff when it comes to these things.