ACR X CLB TO OCCUPIED ALT HAD LTSS FROM ACR Y. SYS ERROR.

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ACR Y; A DH8 (TCASII EQUIPPED); CHKED ON MY FREQ. WAS APPROX 20 NM NW OF AML; LEVEL AT 7000 FT AND WAS NAVING DIRECT AML; DIRECT DCA; LNDG DCA. ACR X; A BE02 (NON- TCASII); CHKED ON MY FREQ. X WAS DEPARTING RWY 19L AT IAD ON A TWR ASSIGNED 240 DEG HDG; CLBING TO 4000 FT. THE DEVELOPMENTAL CTLR INSTRUCTED X TO TURN R HDG 010 DEGS. I OBSERVED X NBOUND; APPROX 5 NM SW OF AML CLBING OUT OF 6200 FT. AT THIS TIME Y WAS 8-10 NM NW OF AML. I INSTRUCTED THE DEVELOPMENTAL CTLR TO ISSUE AN IMMEDIATE DSCNT TO X; AN IMMEDIATE CLB TO Y AND TO EXCHANGE THE TFC. Y ACKNOWLEDGED THE TFC IN SIGHT. X PASSED SLIGHTLY W (BEHIND Y). THE RADAR OP AT IAD REQUIRES CONSTANT COORD BTWN SECTORS; BWI AND DCA APCH AND ZDC. IN THIS INSTANCE I ALLOWED THAT COORD TO DIVERT MY ATTN FROM THE INSTRUCTIONS THE DEVELOPMENTAL CTLR WAS ISSUING. DUE TO THE TIMELY NEED FOR THIS COORD AND THE DEVELOPMENTAL CTLR'S LOW HRS ON HIS FIRST RADAR POS I DECIDED IT WOULD BE MORE EFFICIENT FOR ME TO DO THE COORD FROM THE OVERHEAD POS; PASS THE INSTRUCTIONS TO THE DEVELOPMENTAL; AND THEN EXPLAIN THE PROCESS IN DETAIL DURING OUR TRAINING DEBRIEF. SUPPLEMENTAL INFO FROM ACN 270622: AFTER DEPARTING RWY 19 AT IAD; ACR X BEGAN THE SID; WHICH CALLED FOR FLYING RWY HDG AND CLBING TO 4000 FT. AFTER CONTACTING IAD DEP WE WERE ASSIGNED A SW HDG OF APPROX 240 DEGS. SHORTLY AFTER; WE WERE ASSIGNED A N HDG OF APPROX 010 DEGS AND TOLD TO CLB AND MAINTAIN 7000 FT. JUST AS WE REACHED OUR ASSIGNED ALT OF 7000 FT IAD DEP TOLD US TO MAKE AN IMMEDIATE DSCNT TO 6000 FT DUE TO TFC IN OUR 12 O'CLOCK POS; ALSO AT 7000 FT. AS WE WERE ABOUT TO INITIATE THE DSCNT BOTH MY FO AND MYSELF SAW THE TFC. IT WAS A DASH 8 AHEAD AND SLIGHTLY TO OUR L. THE DASH 8 WAS XING FROM OUR L TO R AND APPEARED TO BE DSNDING. WE STARTED A L TURN AND DID NOT BEGIN OUR DSCNT UNTIL THE DASH 8 HAD PASSED. DSNDING IMMEDIATELY WOULD HAVE DECREASED THE SEPARATION IF NOT CAUSED A COLLISION. AFTER THE DASH 8 PASSED WE DSNDED AS INSTRUCTED TO 6000 FT. AT THE TIME OUR COURSES CROSSED; I ESTIMATE WE WERE APPROX 150 FT ABOVE THE DASH 8. WE ALSO WERE SLIGHTLY BEHIND THE DASH 8; POSSIBLY 1/4 MI OR LESS. SHORTLY AFTER PASSING THE DASH 8; ATC TOLD US IT WAS AN OPERROR ON THEIR PART AND IT WOULD BE LOOKED INTO.