If the Network Analyzer have not enough dinamic range, you may use CW signal generator and sinthetised spectrum analyzer. phase lock toghether the freq. ref., in this manner a 100dB+ dinamic may be reached.
But... to see the antenna mutual coupling, you must be on anechoic enviroment.
Also what "mutual" exactly means? co-pol?, cross-pol? or what? perhaps isolation?
If you expect to measure extremally low level, a significally amount of the signal may came from a received echo of a signal transmitted by adiacent antenna.