Probably, he wants o see how out of phase excitation works on "coplanar". I guess, it is a bad idea and can not be called a coplanar, rather just two slotlines (fed by a hybrid maybe?). So first, you need a model of a hybrid coupler.
That is not a coplanar line, first of all. But anyway, it is not my project, and everyone are free to try and see.
// in coplanar, the mode itself is driven by the central conductor. So it is not just two slots, but a hot coax wire with the jacket unfolded into two infinite planes connected together somewhere beyond physically, or virtually, it does not matter. therefore, in the wave port, you define just one integration line starting from only one hot strip. The only possible wave solution will automatically produce the opposite field vector in the paired slot. Or it may produce a microstrip mode in the solved model of the ground backed cpw, if matching is bad.