Antenna transmitter and receiver in HFSS

This question has been asked before, but really no firm answer has been given from what I have seen in the posts.

Question: I need to simulate 2 antennas and measure the S21 between them.
Do I excite both antennas or just one considering both are matched?
What am I measuring if I excite both and what am I measuring if I just excite one?
Also, if I need to measure the power received by one, is the formula Pacc x S21^2?

I would appreciate direct answers to my questions without going into much theory.

Thanks.