Hi,
I do not know if the thread is still open but wanted to jump in and make a statement or three. My name is Barry LeBlanc, Sandia Aerospace Representive. Here is a AC20-165A link. I have cut and posted some of the high lights.
FAA Advisory Circular AC No: 20-165A found on line at:
http://www.faa.gov/documentLibrary/m...%2020-165A.pdf

3-2. ADS-B Equipment.
a. Equipment eligibility. ADS-B equipment must meet the requirements specified in TSO-C166b or TSO-C154c.
b. Installation guidance.
(1) UAT systems with Mode S transponders. Do not install a UAT ADS-B OUT system which has the capability to transmit a random 24-bit address in an aircraft which also has a Mode S transponder unless the random 24-bit feature is disabled. The ATC automation system would interpret the different 24-bit addresses as two separate aircraft, and alert controllers to a conflict that does not actually exist.
(2) Mixed transmit/receive classifications. TSO-C166b and TSO-C154c allow Class A transmit-only and Class A receive-only equipment configurations. There are no restrictions for installing a certain class of receive equipment with a different class of transmit equipment. For example, a Class A3 transmit-only unit can be used in the same aircraft with a Class A1 receive-only unit. It is also acceptable to have a TSO-C166b transmitter and a TSO-C154c receiver and vice versa.
(3) Stand alone 1090ES transmitters. RTCA/DO-260B, paragraph 2.2.2.2 only allows Class A0 and B0 1090ES stand-alone (not integrated with a transponder) transmitters. This AC does not cover installation approval for class A0 or B0 1090ES transmitters because they are not compliant with 14 CFR 91.227.

(4) Multiple ADS-B OUT systems. If the aircraft has the ability to operate a 1090ES and a UAT ADS-B OUT system at the same time, the systems must have a single point of entry for the emergency code, IDENT, and Mode 3/A code. Neither system may use a random address feature. If dual ADS-B OUT systems of the same link are installed (e.g., to increase dispatch reliability), the installation must preclude operation of both systems simultaneously

Note: 1: We recommend that you do not install both 1090ES and UAT ADS-B OUT capability on the same aircraft.


I have more to add. But my email is barry.leblanc at sandia.aero
Look forward to hearing from you why not a MODE C Transponder?