Wall join in corner not clean

[sarahric]

I have 2 walls that join at a corner. The building materials used in the composite walls are the same, so why don't each of them join up?
I can only get 1 material to join by making it the strongest.
Reference lines join, clean intersections turned on.

[sarahric]

Ok, I see.
I didn't consider making duplicates of materials, but see that I may need that for have external and internal brickwork.
Thanks Barry.