If you want to design a coupler for instance with a broadside coupler or a coupled line (according to the coupling you desire) you have to take into account the different velocitu of odd mode and even mode. So if you want good directivity even with these structures you have to make equal the two velocity. Usually one can do this adding a capacitor at the beginning and at the end of the lines or even in the middle.
The equation are the same of the coupled lines
hoping this can help u
stefano