Francis...I...will...print ...this....sloooowly....so...you ....may...understand...what...I...am....trying....to...show. ..you.
ADS-B UAT is not seen by TCAS hence the need to retain a tranponder...1090ES is ALREADY a mode ACS transponder, although some are now coming out in Europe as modeS ES...made S can still be seen by TCAS...YOU ONLY NEED THE ONE UNIT!