Thanks all!
@Volker
I've tried out your suggestions. I set the port impedance in order to minimize S11 and maximize S12. With that values I can now calculate the characteristic impedance matrix ele ments with the equations:
\[{Z }_{diff } = 2({Z}_{0,11}-{Z}_{0,12})\]
\[{Z }_{comm} = 1/2({Z}_{0,11}+{Z}_{0,12})\]
Finally, I have the complete characteristic impedance matrix (symmetric structure):
\[{Z }_{0} = [{Z}_{0,11}\, {Z}_{0,12}\\
\quad \quad \quad \quad \quad \quad \quad {Z}_{0,12}\, {Z}_{0,11}]\]
Is that right?