mutual coupling waveguides

Hi everyone,

I am trying to model two oppositely located waveguides for heating fluids. I am using 4.2, Electromagnetic waves in RF module and non-isothermal module in heat transfer module.
I am trying to simulate whole model in 3D.

I am using following boundary conditions for emw;
Port (for mutual coupling waveguides)
Scattering boundary condition (for inlet and outlet)
Impedance boundary condition (metal boundaries)

When I run the program I had a wrong solution. Because of mutual coupling the waves can interfere each other (I am not exactly sure it is because of that) . But I don´t know what will I do about this. I must use boundary conditions except these boundary conditions or something else?

I couldn´t find any topic about this. Can anyone help me please?

' I attached my model in attachements.