MEM APCH CTLR EXPERIENCED OPERROR AT 3000 FT WHEN VECTORING ACFT TO FINAL VIA SIMULTANEOUS BASE TURNS.

Narrative

I WAS WORKING W FINAL POS ON MY 14TH SHIFT IN 16 DAYS. ACR X; A CRJ2; WAS ON DOWNWIND FOR RWY 36L. ACR X WAS UNPRESSURIZED; SO I HAD TO EXTEND HIS DOWNWIND SUCH THAT WHEN I BASED AND TURNED THE ACFT TO FINAL THEY WERE ABOUT 20 MI FROM THE ARPT. ACR Y; ALSO A CRJ2; WAS ON DOWNWIND AND I DSNDED THEM TO 2000 FT AFTER THEY CLRED A 4500 FT VFR OVERFLT -- MY PLAN BEING TO PUT ACR Y IN FRONT OF ACR X. EARLIER; WHILE ACR X WAS ON DOWNWIND; I NOTICED THEIR SPD TO BE ABOUT 210 KTS. I BASED ACR Y A LITTLE LATE; BUT STILL THOUGHT I HAD ROOM TO GET THAT ACFT IN FIRST. ACR X WAS NOW AT 3000 FT ON FINAL. AS MY PLAN BEGAN TO LOOK WORSE AND WORSE; I INSTRUCTED ACR Y TO EXPEDITE TO 2000 FT. ACR Y REPLIED THAT THEY WERE ALMOST THERE. I TURNED ACR Y TO JOIN THE LOC FOR RWY 36L. I THEN POINTED OUT TFC TO ACR X AND ASKED IF THEY HAD TFC IN SIGHT. THEY DIDN'T. AT THAT POINT; I NOTICED THAT ACR X'S SPD WAS INDICATING 270 KTS. I HAD NEVER HAD AN ACFT INCREASE SPD DURING ANY PHASE OF THE ARR PROCESS WITHOUT MY INSTRUCTION BEFORE. I REALIZED THAT MY PLAN WAS NOT GOING TO WORK; SO I BROKE ACR X OUT TO THE W WHEN ACR Y HAD REACHED 2000 FT. UNFORTUNATELY; I HAD LOST LATERAL SEPARATION PRIOR TO THAT. I DO NOT KNOW WHY ACR X INCREASED THEIR SPD; BUT I WILL BE CERTAIN TO LOOK FOR THIS IN THE FUTURE.