It is not necessary to unite them.
Yes when I talk about outer box, I am refering to the box where the Radiation Boundary is assigned and where your cpw will be placed inside. The subtraction will leave some "empty space". This empty space will be filled with air if the outer boxes material is vacuum.
You should add it only in the curved face. The top and bottom circles should touch your copper conductors so there is no need to set Finite Conductivity there.
You are navigating away from the AIS Discovery experience