HFSS manual on Master slave boundary conditions?

can master boundary be applied to one face of air box and slave on the opposite phase in hfss or we need separate master air box and slave air box??

You do not need separate air boxes. They both can be applied to the same airbox or you can draw a rectangular sheet on both sides and apply them to two seperate sheets. I have done this a lot and it works very well. It can be tricky to select the faces sometimes in the rendered 3D view of your model, so sometimes it might be easier to select the objects or faces by name ("Edit", "Select", and then "by name") or you can select whole object in the model tree for your different solid and sheet objects. Hope that helps!