Actually, i am not sure if i understand your question. Are A and B at the same frequency, 10GHz? If so, for the getting C=A-B, do you already know the phase of the two signals, A and B? If you use extra λ/2 length of transmission line, you have to make sure they have the same phase before the signals are input to this network. It is the same case for you to "add" the two signals. You only need to combine, by a Wilkinson power divider, the two signals with 180 out of phase or in phase when you want they are substracted or added, respectively.