If you observe a good VSWR in EM simulation, you are done. Because you have a coupled structure, you have even and odd mod impedances. Driving the ports with 90 degree shift is neither even nor odd mode, I do not know whether a analytical description is available, EM solution may be the only way. For the coupled structures, excitation scenario may significantly change the results, you need to provide the same excitation scenario that you intent to use while simulating the structure.
Are the conventional z-parameters applicable for a coupled structure? Even and odd mode cases must be considered to find correct port parameters.