ACR XA HAD LTSS FROM ACFT Y. SYS ERROR.

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CONDUCTING OJT ON RADAR TRAINEE AND LET TRAINEE GO TOO FAR. WHEN I ATTEMPTED TO STOP ACFT TO MAINTAIN 1000 FT SEPARATION I USED WRONG CALL SIGN AS THEY WERE SIMILAR. SAME COMPANY ACR XA ACR XB ALL ON FREQ. CHANGING CALL SIGN NUMBERS MIGHT HAVE PREVENTED CONFUSION. ACR XA HDG EBOUND AT FL230 WANTS LOWER. ACFT Y AT 10 O'CLOCK AND 15 MI SEBOUND DSNDING TO 150 DSNDING OUT OF 220 IS TFC. ACR XB GIVEN A 190 VECTOR AND DSNDED TO 110. TURN WAS NOT GOING TO BE SUFFICIENT. I WAS WAITING FOR TRAINEE TO FIX PROBLEM. I WAS GOING TO STOP ACR XA'S DSCNT TO MAINTAIN 1000 FT SEPARATION BUT I SAID ACR XB INSTEAD. ACFT Y TURNED TO A 110 HDG AND TOLD TO RPT ALT LEAVING BUT ONLY SAID 20700 AND ACR XA WAS AT 21500. BAD JUDGEMENT INVOLVED ON MY PART IN LETTING TRAINEE GO TOO FAR. ALSO WHEN IMMEDIATE ACTION WAS TO BE TAKEN I CALLED SAME COMPANY FLT BY WRONG NUMBER USING ACR XB. SUPPLEMENTAL INFO FROM ACN 197822: SPACING WAS BEING ISSUED FOR SDF ARRS THROUGH THE GATE. SAME COMPANY ACR'S WERE THE FIRST 2 FOLLOWED BY ACFT Y AND ACR XA. ACR XB WAS GIVEN A CLRNC TO MAINTAIN 180 TO KEEP HIM FROM DSNDING INTO ACR XB. MEANWHILE ACFT Y WAS GIVEN A CLRNC TO DSND FROM 220 TO 150 AND CHANGED FROM A 190 HDG TO A 170 HDG. ACR XA THEN ASKED FOR LOWER AND WAS GIVEN A 190 HDG TO DSND TO 110. ACR XA MADE A SHARP TURN TO 190 HDG AND AN INCREASED DSCNT RATE WHICH WAS GREATER THAN ACFT Y. WHEN THE INSTRUCTOR JUMPED IN TO STOP ACR XA'S DSCNT HE MISTAKENLY USED ACR XB'S CALL SIGN. ACR XB RESPONDED HE WAS LEVEL AT 180. THE TRAINER THEN RESTATED FOR ACR XA TO STOP HIS DSCNT AND SAY ALT LEAVING. AT THIS TIME SEPARATION HAD BEEN LOST.