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

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I WORKED 2 HRS AND 16 MINS (I HAD TOLD THE SUPVR AT 2 HRS I NEEDED A BREAK) AT IMPERIAL SECTOR WITH EXTREMELY HVY TFC; TOOK 24 MIN BREAK. WORKED 22 MIN IMPERIAL 'D' SIDE; THEN WENT TO 2 RADAR FOR 40 MINS OF VERY COMPLEX TFC BEFORE THE OCCURRENCE. I HAD 3 WBOUND JETS AT FL310; 1 NBOUND JET AT FL290. PLUS ABOUT 10 OTHER ACFT ON MY PVD. TFC WAS COMPLEX DUE TO TURB. ACFT WANTED LOWER ALTS; CAUSING A LOT OF XING AND TRANSITIONING. NBOUND ACR X FL290 WAS ISSUED TFC ON ACR Y 2 O'CLOCK 10 MI WBOUND AT FL310; THEN I CHANGED THE COMS TO 120.02 AND DROPPED THE FULL DATA BLOCK AS THE ACFT LEFT MY AIRSPACE. THEN AN ACFT DEVIATED FROM ALT ASSIGNMENT AND RTE. NEXT WBOUND ACR Y REQUESTED LOWER FOR CHOP. (I HAD PREVIOUSLY OBTAINED CTL FOR LOWER ON THE 3 WBOUNDS.) I HAD FORGOT ABOUT ACR X AND COULDN'T SEE THE LIMITED DATA BLOCK BECAUSE ACR Y FULL DATA BLOCK WAS COVERING IT. I DSND ACR Y TO FL280. A MIN LATER THE CONFLICT ALERT WAS ACTIVATED. I CLBED ACR Y BACK TO FL310; ACR Y SAID HE WAS AT FL300; SHOULD HE CLB? I OBSERVED THE TARGETS DIVERGING BY ABOUT 90 DEG AND 3 MI; AND THERE WAS NO DANGER. SO I TOLD ACR Y TO CONTINUE TO FL280 AND ISSUED TFC. ACR Y ADVISED HE HAD BEEN WATCHING THE TFC; AND WAS ALSO ON TCASII. HAD I NOT BEEN SO EXHAUSTED; THIS MAY HAVE BEEN AVOIDED. SUPPLEMENTAL INFO FROM ACN 189476: NBOUND ACR X AT FL290 EXITING AIR SECTOR'S AIRSPACE. WBOUND ACFT AT FL310 ASKED FOR DSCNT DUE TO CHOP. ACR Y WAS DSNDED THROUGH THE AIRSPACE OF THE NBOUND ACFT. THE EVENING WAS A VERY BUSY ONE WITH LONG SHIFTS AND FEW BREAKS WITH A DIFFICULT WORKLOAD.