Occasionally, the two aircraft declare each other as threats simultaneously, and therefore, both aircraft will select their RA sense based on the encounter geometry. In these encounters, there is a chance that both aircraft will select the same sense. When this happens, the aircraft with the higher Mode S address will detect the selection of the same sense and will reverse its sense.
Version 7 includes the capability for TCAS to issue RA reversals in coordinated encounters if the encounter geometry changes after the initial RA is issued. The RA reversals in coordinated encounters are annunciated to the pilot in the same way as RA reversals against non-TCAS intruders. In a coordinated encounter, if the aircraft with the low Mode S address has Version 7 installed, the low Mode S address can reverse the sense of its initial RA and communicate this to the high Mode S address aircraft. The high Mode S address aircraft will then reverse its displayed RA. The aircraft with the high Mode S address can be equipped with either Version 6.04 or Version 7.
In a coordinated encounter, only one RA reversal based on changes in the encounter geometry can be issued. The initial RA sense will not be reversed until it has been displayed for at least nine seconds, unless the low Mode S address aircraft has a vertical rate higher than 2500 feet per minute and acts contrary to the RA. This delay is included in the design to allow sufficient time for the two aircraft to initiate a response to the initial RA.
http://www.arinc.com/downloads/tcas/tcas.pdf