DURING CRUISE AT FL310; AN MD80 RECEIVES A TCASII ALERT AND SPOT A DSNDING ACR XING L TO R 1 MI AHEAD.

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WE RECEIVED A TCASII TFC ALERT FOLLOWED BY AN RA. AT THE TFC ALERT I LOOKED OUT AND SAW AN ACR Y ACFT; I BELIEVE IN THE CAPT'S FRONT SIDE WINDOW. WE WERE APPROX 60 NM NE OF ORMOND BEACH (OMN 040 DEG RADIAL AT 60 DME). AT THE TFC ALERT; THE TCASII SHOWED THE ACFT 800 FT ABOVE US. TCASII QUICKLY DELIVERED AN RA TO 'DSND.' JUST PRIOR AND AT THE START OF THE RA; TCASII SHOWED ACR Y AT 700 FT ABOVE US. AFTER THE RA; I CONCENTRATED ALL OF MY SCAN ON THE ACR Y ACFT. CAPT INITIATED A DSCNT AFTER DISCONNECTING THE AUTOPLT AND AUTOTHROTTLES. I IMMEDIATELY ALERTED ZJX THAT 'ACR X IS DSNDING IN RESPONSE TO AN RA!' I BELIEVE THE CTR RESPONDED WITH 'WHO'S DSNDING?' I RESPONDED AGAIN WITH 'ACR X IS DSNDING IN RESPONSE TO AN RA.' CAPT THEN FOLLOWS TCASII'S CALL TO 'ADJUST VERT SPD' FOLLOWED BY 'CLR OF CONFLICT' AND CLBS BACK TO FL310. ZJX ASKS ACR Y (SOMEWHERE IN THE MIDDLE OF ALL THIS) 'ACR Y SAY YOUR ALT!' AND ACR Y RESPONDS 'ACR Y IS CLBING FROM OUR DSCNT IN RESPONSE TO AN RA.' ACR Y IS LATER GIVEN CLRNC TO CONTINUE THEIR DSCNT TO FL280. I WOULD GUESS THAT OUR NEAREST POINT OF APCH IS 500-700 FT VERTLY AND INSIDE 1 NM HORIZLY.