Unfortunately there is no unify wall command.
The best you can do is stretch the end of one wall to the end of the other and delete the shorter wall.
This of course will delete any doors & windows in that wall.
The trick there is to mark the location of the windows with a hotspot.
Now drag the short wall (the one you want to delete) to the side going along the length of the wall until it is just clear of the longer wall (the one you want to keep).
Then just drag the doors & windows from the short wall back into the long wall.
So long as the reference lines are in line with each other you can drag the wall from one wall to the other.
Now delete the short wall (that has no doors or windows left).
A bit long winded but it does work.
Generally I wouldn't bother unless you had a door or window spanning the join between both walls and even then I would just stretch the ends of the walls to one side of the window.
Barry.
P.S. you can also drag copies of doors and windows and you can also do it in 3D or elevation vertically to drag or copy windows from one flor to another.
So long as the wall reference lines are in the same plane - they can even be different wall composites.
One of the forum moderators.
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