Sounds like you want a wall and at the same time no wall
Try it with levels: on the base level you have no wall where you want the opening with the height of the walls up to where the top of the open wall starts. On a level higher you draw all walls including the tops of the openings. By changing the level heights and elevations you should be able to get what you want: The base level shows no wall in the opening and in the 3D view you can "Show all levels" to display both the lower part of the walls and the top wall parts for the opening.
There's one disadvantage to this: you might see the cutoff line of some textures between the two wall heights.
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