What's the effect of S21 in two port antenna system?
S21 presents how much there's interconnection between the ports. If we assume you have in port 1 a radio transmitter and the receiver is connected to port 2 you receiver can have hard times if S21 is only minus few decibels.
If both ports have a receiver some problems can arise from high  S21, at least if one of the receivers is not well matched. You can get frequency dependent attenuation. I think -6...-10dB is meaningless.
If both ports have a transmitter and there's near zero S21 some malfunction can be caused. Leaked strong signal can make the output stage of the other transmitter to a mixer which operates at high signal levels.