Material join within a wall

[Bryan Thatcher]

Whether I use a stacked wall or a wall with multiple layers, I get a line dividing the common material. Is there any way to fix this? Thanks.

[cliff collins]

Two walls, Join Geometry?

Last resort: Linework tool, invisible lines.

[Mike Sealander]

It appears you are trying to create a shelf in your concrete wall. Maybe try using a void extrusion?