CVG LCL CTLR OBSERVED NMAC WITH TFC ON BASE FAILED TO INTERCEPT FINAL FOR RWY 18L AND CONFLICTED WITH TFC ON FINAL FOR RWY 18R.

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ACR X ON BASE FOR RWY 18L (CVG). ACR Y ON STRAIGHT-IN RWY 18R. ACR X CALLED ME AT LCE. I CLRED HIM TO LAND. CONFLICT ALERT ACTIVATED AS ACR X MISSED HIS TURNOFF BASE TO FINAL. ACR X WENT THROUGH FINAL. LCL CTLR ISSUED TFC; AND ACR Y WAS INSTRUCTED TO MAINTAIN VISUAL WITH ACFT. I TOLD ACR X TO IMMEDIATELY RETURN TO HIS FINAL; AND HE ADVISED TFC IN SIGHT. MODE C INDICATED 100 FT DISTANCE; AND PRIMARY TARGETS MERGED. ONCE ACFT X WAS ON THE GND; I ASKED WHAT HAPPENED. HE SAID THE AUTOPLT FAILED TO INTERCEPT THE LOC. I TOLD HIM HE MADE A LOT OF PEOPLE NERVOUS WITH THE MANEUVER AND ASKED THAT NEXT TIME HE 'GRAB THE STICK' WHEN HE REALIZES SOMETHING'S NOT RIGHT. HE ACKNOWLEDGED WITH HIS CALL SIGN ONLY. BEING ONLY A CTLR; I DON'T SPEAK WITH MUCH KNOWLEDGE OF THE AUTOPLT SYS; BUT I CAN'T BELIEVE IT'S AN ACCEPTED PRACTICE TO ALLOW THE ACFT TO TURN ITSELF TO FINAL AT +190 KTS; 90 DEGS FROM THE RWY. THE PLT SHOULD HAVE RECOGNIZED 1) PARALLEL APCHS MOVE; 2) THE AIRPLANE ON THE R SIDE LOC; AND 3) THE LACK OF RESPONSE FROM THE AUTOPLT AND MADE SURE KIND OF STICK INPUT PRIOR TO XING THE LOC.