The two antennas are there because the airframe would otherwise shield the radio signal causing blind spots for TCAS, Radar and ADS-B. The transponder replies to interrogations through the antenna on which the strongest interrogation signal was received. Squitters (transmission unsolicited by interrogations used for TCAS and ADS-B) are send more or less randomly out of either the top or bottom antenna. It is called "antenna diversity"
Aircraft fitted with a Mode S transponder and with a MTOM > 5700kg or maximum cruising speed > 250 KTAS must have antenna diversity (ICAO Annex 10, vol IV, 2.1.5.3)
As far as I know, there is no requirement to carry two transponders. The reason for carrying two transponders is redundancy; if you wouldn't have a backup system, a transponder failure would mean landing at the nearest suitable airport in most airspaces.
Last edited by ATCast; 30th August 2013 at 08:37.