func illusionary makes a brush you can walk through but cant see throuh (depending on your render settings). the only way is to either make the brush thicker or to make 2 brushes. observe: (im not sure this will work) |=brush
|<*>|
*these faces should be textured null.
you shouldnt need to change the settings of your func wall for this to work, however there must be 2 brushes for this to work.
explained in another way, just copy your func_wall and place it next to the original, then texture the inner faces with the NULL texture. if you have ZHLT compile tools you should have the zhlt.wad which contains the NULL, SKIP and HINT textures.