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

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WORKING DEP RADAR WITH NUMEROUS DEVS FOR WX. ACR X CHKED ON CLBING TO 9000 FT ON A SUMMA 5 DEP. CLBED TO 15000 FT AFTER RADAR IDENT. LATER PLT ASKED TO DEV W FOR WX; ASSIGNED 250 DEG HDG. PLT LATER REQUESTED 220 DEG HDG AND THIS WAS APPROVED. ACR Y INBOUND ON OLM ARR LEVEL 10000 FT. ACR X WAS UNDER INBOUND LEG. ACR X ADVISED COULD TAKE TURN AND ASKED TO CONFIRM ASSIGNED ALT. OBSERVED ACR X CLBING THROUGH 9200 FT. ISSUED 9000 FT A TURN TO 120 DEG OF TFC AT 12 O'CLOCK; 3 MI; 10000 FT. SEPARATION LOST. ACR X SHOULD HAVE BEEN STOPPED IMMEDIATELY AT 9000 FT WHEN DEV REQUESTED DUE TO PROX TO INBOUND LEG. A TFC CALL AT EARLIER TIME COULD HAVE CUED A REITERATION OF ALT AND CORRECTED (PREVENTED) SIT. SUPPLEMENTAL INFO FROM ACN 273729: FO FLYING ON DEP OUT OF SEATAC ACR X CLRED TO 9000 FT. AT ABOUT 5 DME FROM SEATAC I ASKED AND RECEIVED DEV W FOR WX. AT SOME POINT THE FO THOUGHT HE HEARD US CLRED TO 15000 FT IN ASSOCIATION WITH A TURN. HE THEN ARMED 15000 FT. I DIDN'T NOTICE THIS TILL ABOUT 8000 FT. I ASKED HIM ABOUT IT AND THEN CALLED ATC TO VERIFY AN ASSIGNED ALT. WHEN THEY DIDN'T RESPOND RIGHT AWAY; I PUSHED ALT HOLD. ATC THEN SAID WE WERE ASSIGNED 9000 FT AND SHORTLY AFTER TOLD US TO TURN L FOR TFC AT ABOUT THE SAME TIME OUR TCASII ANNOUNCED 'TFC.' I ALSO GOT A VISUAL ON THE ACR Y. I THEN TOOK CTL OF THE ACFT AND UNCOUPLED THE AUTOPLT AND INCREASE THE BANK ANGLE. DURING THESE MANEUVERS THE ACFT CLBED AS HIGH AS 9400 FT. I ALSO THINK THAT IN MANEUVERING TO AVOID THE ACFT; I HAD VISUAL I MAY HAVE CLBED A COUPLE OF HUNDRED FT. AFTER LNDG TALKED TO ATC IN SEA.