PCT CTLR DESCRIBED CONFLICT AFTER COMBINING DEP AND ARR POSITIONS WHEN RELIEVED CTLR FAILED TO BRIEF AND DESCEND ONE OF TWO ARR ACFT.

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I COMBINED THE ARR SECTOR TO MY DEP SECTOR AND RECEIVED THE BRIEFING FROM THE ARR CTLR. THE ARR CTLR TOLD ME ABOUT VERY STRONG WINDS FROM THE SW AND THAT HE WAS XING OVER THE TOP OF THE LOC AND TURNING INTO THE L DOWNWIND TO BETTER SLOW THE ACFT TO MAKE THE SEQUENCE ON TO FINAL. HE BRIEFED ME ON ABOUT 6 ARRS INCLUDING THE 2 ACFT IN QUESTION; ACR X AND ACR Y. HE TOLD ME ACR Y WAS ESTABLISHED ON THE LOC LEVEL AT 5000 FT. HE TOLD ME ACR X WAS OUT OF 8000 FT FOR 6000 FT AND SINCE ACR X WAS FROM THE SW I AGREED WITH HIM ABOUT XING OVER THE TOP OF FINAL. HE PUT ACR X ON A 090 DEG HDG TO GO OVER THE TOP OF ACR Y. I THEN KEYED UP THE FREQS AND MY FIRST XMISSION WAS TO DSND ACR Y TO 3000 FT. THE ARR CTLR WALKED DOWN TO GIVE ME HIS ARR STRIPS WHEN WE BOTH HEARD THE CONFLICT ALERT ALARM. I NOTICED ACR X DSNDING THROUGH 5800 FT AND ACR Y DSNDING THROUGH 4700 FT. I IMMEDIATELY CLBED ACR X BACK TO 6000 FT AND EXPEDITED ACR Y THROUGH 4000 FT TO MAINTAIN 3000 FT WHILE SEPARATION MIGHT HAVE BEEN LOST AT THAT POINT. THE CTLR THEN TOLD ME THAT HE FORGOT THAT HE DSNDED ACR X TO 5000 FT. CONTRIBUTING FACTORS: DUE TO FORCED OVERTIME; THIS IS MY 12TH DAY WORKED OUT OF THE LAST 13 DAYS. I MIGHT HAVE CAUGHT THE DSCNT RATE BETTER ON ACR X. THE ARR CTLR WAS GOING HOME WITHIN 10 MINS SO HE WAS RUSHING. OUR AIRSPACE (MTV SECTOR) ON A S FLOW IS VERY SMALL AND COMPACT TO SEQUENCE THE ARRS; WE BUMP UP AGAINST IAD AIRSPACE; THERE IS NO MARGIN FOR ERROR. I ALSO BELIEVE THAT THE SUPVR SHOULD HAVE STAYED LONGER SINCE WE HAD A HVY ARR FLOW DUE TO BAD WX EARLIER. THE BOTTOM LINE: THE ARR CTLR SHOULD HAVE DSNDED ACR Y AND STOPPED ACR X AT 6000 FT.