A window and its associated hole can only belong to one wall.
So it is a case of Solid Element Operation as you suggest or place an empty opening in the wall below for the sill - either way you have two elements to adjust if the window changes.
Or use a single wall as you mentioned which means you only have to place the one window.
If the wall spans more than one storey you can make it display on the home storey only or all relevant storeys.
Barry.
One of the forum moderators.
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