Guilin Sun
Ansys Employee

If they are not the same degenetated modes, maybe I am wrong but I suppose they are incohent. However simulation is coherent. This might be contradictory. This will need you to justify, as this is a theoretical issue.

If you really want to do the simulation,it might be good to do two separate simulations. But seems you only want to do one simulation. If you think they are coherent, you can manually calculate their source power, by adding the E and H fields coherently, calculating the Poynting vector, and integrating it. Please refer 

Ansys Insight: FDTD 初学者: transmission函数的使用、透射率反射率正负号问题