ACR X TCASII RA SAME ALT ASSIGNED HAD LTSS FROM ACR Y. SYS ERROR.

Narrative

ACR X WAS BEING HANDLED BY ATL APCH CTL AND BEING FLOWN BY THE FO. AUTOPLT WAS ENGAGED AND WE WERE ON A DOWNWIND LEG FOR RWY 9R ON A 280 DEG HDG. INITIALLY; OUR ASSIGNED ALT WAS 7000 FT; BUT WHILE DSNDING OUT OF ABOUT 8500 FT; THE CTLR AMENDED OUR ALT ASSIGNMENT TO 8000 FT; WHICH WE COMPLIED WITH. THIS AMENDMENT CAUSED US TO BE WARY OF CONFLICTING TFC. ALMOST IMMEDIATELY; WE RECEIVED A TA ON OUR TCASII; ABOUT OUR 10 O'CLOCK POS ON A CONVERGING COURSE AND DSNDING TOWARD OUR ALT. ABOUT THE TIME THE CAPT SPOTTED THE TFC VISUALLY; WE RECEIVED A TCASII RA COMMANDING A DSCNT. I DISCONNECTED THE AUTOPLT AND BEGAN A DSCNT TO COMPLY WITH THE RA. AT VIRTUALLY THE SAME TIME I BEGAN THE DSCNT; THE CTLR DIRECTED US TO TURN TO A 300 DEG HDG AND GAVE THE CONVERGING ACR Y A L TURN TO A 250 DEG HDG AND TOLD HIM TO STOP HIS DSCNT. I IMMEDIATELY COMPLIED AS DID ACR Y. THE RA CEASED AND A 'CLR OF CONFLICT' WAS HEARD. WE HAD ONLY DSNDED ABOUT 200 FT. UPON LNDG THE CAPT CALLED APCH CTL WHEREUPON HE LEARNED THAT ACR Y INBOUND FROM THE SW HAD BEEN CLRED TO 5000 FT AND THAT ACCORDING TO APCH'S RADAR WE HAD COME WITHIN 1.2 MI AND 200 FT ALT SEPARATION OF THE OTHER ACFT. SUPPLEMENTAL INFO FROM ACN 250159: PHONE CALL TO ATL APCH CTL SUPVR. THE SUPVR INFORMED ME THAT HE HAD 'PULLED THE TAPES' AND HIS INITIAL OBSERVATION OF THEM CONFIRMED MY ACFT'S ACTIONS AS WELL AS THOSE OF THE CONFLICTING TFC. HOWEVER; HE STATED THAT HE HAD NOT HAD TIME TO THOROUGHLY ANALYZE THE TAPES. THE SUPVR THEN SAID THAT HE WOULD BE FILING AN 'OPERROR RPT' DUE TO THE CLOSE PROX OF ACFT INVOLVED. THE SUPVR INFORMED ME THAT THE 'TAPES SHOWED THE ACFT TO BE SEPARATED 1.2 MI HORIZLY AND 200 FT VERTICALLY.' OUR COURSES WERE DEFINITELY CONVERGING AND EVASIVE ACTION WAS REQUIRED TO AVOID A 'NEAR HIT.' THAT WDB LOOKED PRETTY IMPRESSIVE UP UNTIL THE TIME THAT OUR COURSES BEGAN TO DIVERGE.