The only way I could get it to work is by making two half walls. On the left is a single wall with the double core, and on the right are two walls placed back to back (as you can see by the selection handles).
You will need to hide the outline on the back side in the composite's definition (and be sure to set the walls to use the composite's pens & lines).
I wouldn't want to do much of this in a project, but if it's just the party walls (presumably with few or no penetrations) it shouldn't be too much trouble.