3 Sided Fireplace.vwxHi There,
I'm late to the party but found this post helpful with my same problem.
I was attempting a 3 sided fireplace but also encountered the same issue with a double sided.
My solution was to make a custom wall thickness that included the cavity for the fireplace (20" wall thickness)
Like Robert suggested above, I made the insertion point in the center of the symbol.
I included my working file for example. Seems like its a good solution