SCT DEV CTLR DESCRIBED NMAC WHEN AN IFR ACFT; ISSUED VISUAL SEPARATION CLIMB CLRNC BUT SEEING WRONG ACFT; CONFLICTED WITH ANOTHER IFR ACFT.

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I WAS TRAINING ON 2 COMBINED RADAR SECTORS. THE CONQUEST WAS RELEASED FROM LGB AND TOOK OFF ON THE ASSIGNED HDG OF 200 DEGS CLBING TO 3000 FT. I ADJUSTED THE HDG TO 180 DEGS AS WITH MOST JET DEPS. MY TRAINER AND I BEGAN TO DISCUSS HOW THE HDG OF 200 DEGS WAS BETTER FOR VARIOUS REASONS. THE C172 WAS TRACKING WBOUND ON A HDG TO JOIN A RADIAL AT 4000 FT. SINCE I NEEDED TO CLB THE CONQUEST; I ISSUED TFC; WHICH HE RPTED IN SIGHT; AND I INSTRUCTED HIM TO MAINTAIN VISUAL SEPARATION; WHICH HE ACCEPTED. I ISSUED TFC TO THE C172; WHICH HE ALSO RPTED IN SIGHT. THERE WAS ANOTHER VFR TARGET AT 3500 FT; WHICH I OBSERVED; TRACKING EBOUND; THAT MUST HAVE BEEN ON THE C425'S TCAS; BECAUSE HE SAID HE WOULD HOLD 2400 FT BRIEFLY. I DID NOT UNDERSTAND WHY; AND REITERATED THE CLB TO 7000 FT; SINCE THE TFC WAS NOT A FACTOR. HE BEGAN TO CLB TOWARD THE WBOUND C172; AND I BECAME ALARMED WHEN THE TARGETS OF THE C425 AND C172 APPEARED TO BE MERGING RAPIDLY AND NEAR THE SAME ALT. MY INSTRUCTOR SAID; 'CLASSIC CASE OF SEEING THE WRONG ONE.' I STARTED TO KEY; BUT BY THEN IT WAS TOO LATE. BOTH ACFT WERE SHOWING 4000 FT WHEN THE TARGETS MERGED. THEN THE XPONDERS CANCELED; AND I WAS SURE THAT THEY HIT. ON THE NEXT SWEEP; THE C425 WAS AT 4300 FT AND THE C172 WAS AT 3700 FT AND DEVIATING SWBOUND. BY SOME MIRACLE; THEY MISSED. I AM SHAKING AS I WRITE THIS. EVERYTHING ELSE WENT ROUTINELY; BUT I LEARNED A FEW THINGS. FIRST; VISUAL SEPARATION AT NIGHT IS BAD BECAUSE OF THE CONFUSION THAT CAN EASILY OCCUR IF THERE IS OTHER TFC NEARBY. SECOND; I SHOULD HAVE CONTINUED VERT SEPARATION UNTIL I WAS CERTAIN THAT HE HAD THE CORRECT AIRPLANE IN SIGHT. EVEN THOUGH THE TFC WAS ISSUED AS 'WBOUND;' I SOMETIMES CONFUSE THE TWO (AS DO ALL ATCS AT TIMES). THIRD; SINCE THE C425 HAD ACCEPTED VISUAL SEPARATION; IT WAS TECHNICALLY HIS FAULT IF THEY HIT; BUT I CANNOT HELP BUT FEELING THAT I WAS A CONTRIBUTING FACTOR. ALL IN ALL; IT WAS A GREAT LEARNING EXPERIENCE THAT I HOPE NEVER TO SEE AGAIN.