NMAC BTWN A CPR DSNDING INTO THE TERMINAL AREA AND ANOTHER UNKNOWN ACFT IN LEVEL FLT XING OVER. BOTH ACFT WERE IN VFR CONDITIONS.

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WE WERE CLRED DIRECT TO THE ARPT; USING GPS FMS WITH VERT NAV GUIDANCE; BY ZAU. WX CONDITIONS WERE CLR; WITH DARK NIGHT. LEVEL AT ASSIGNED 5000 FT MSL; THE CTLR ADVISED US OF 'TWIN CESSNA TFC; 11:30 O'CLOCK AND 5 MI; SWBOUND AT 4000 FT.' WE LOOKED FOR THE TFC VISUALLY AND ON TCASII; EVENTUALLY SIGHTING IT IN BOTH WAYS. WE ADVISED HAVING THE TFC AND THE ARPT IN SIGHT. SOME SHORT PERIOD OF TIME AFTER RPTING THE TFC IN SIGHT; WE WERE CLRED TO DSND; AT PLT'S DISCRETION; TO 3000 FT. WE ACKNOWLEDGED THIS AND RPTED LEAVING 5000 FT; WITH THE COPLT HAND-FLYING THE ACFT. BOTH OF US WERE WATCHING THE TFC VISUALLY; AND I WAS ALSO REFERRING PERIODICALLY TO THE TCASII. INITIALLY; THE OTHER ACFT APPEARED TO HAVE PLENTY OF RELATIVE MOTION AND TO BE NEARLY PARALLELING OUR COURSE. HOWEVER; AS WE DSNDED THROUGH 4000 FT; THE AIRPLANE RAPIDLY APPEARED TO BE XING OUR PATH; FROM L TO R AND VERY CLOSE (ALTHOUGH IT WAS VERY DIFFICULT TO ESTIMATE HOW CLOSE; BECAUSE OF THE DARKNESS). AT THIS TIME; WE ALSO GOT A TCASII AURAL ALERT. FACTORS LEADING UP TO THE INCIDENT: 1) VISUAL SIGHTING AND INITIAL RELATIVE MOTION IN THE DARKNESS MADE IT APPEAR THAT THE TFC WOULD BE NO FACTOR. 2) CLRNC TO DSND TO 3000 FT LED ME TO BELIEVE THAT ATC ALSO CONSIDERED THE TFC TO BE NO CONFLICT. IN RETROSPECT; THIS WAS PROBABLY A WRONG ASSUMPTION; BECAUSE I HAD RPTED THE TFC IN SIGHT. 3) HVY COCKPIT WORKLOAD DURING DSCNT; AT THE END OF A FAIRLY LONG DAY; AND WHILE I WAS GIVING DUAL INSTRUCTION TO THE COPLT DISTRACTED ME FROM PROPERLY EVALUATING THE ALTS; HEADINGS; AND POTENTIAL FOR CONFLICT THAT WERE CORRECTLY POINTED OUT BY THE CTLR. 4) INITIAL TCASII INDICATIONS ALSO LULLED ME INTO THINKING THAT THERE WAS NO CONFLICT. HAD I NOT HAD TCASII ABOARD; I PROBABLY WOULD HAVE REMAINED AT 5000 FT; WHICH THE 'PLT'S DISCRETION' CLRNC WOULD HAVE ALLOWED ME TO DO. SUGGESTIONS FOR PREVENTION: 1) CTLR: WITHHOLD ANY TYPE OF DSCNT CLRNC FOR IFR ACFT WHEN POTENTIAL XING ALTS EXIST. IF REDUCED SEPARATION BECOMES EVIDENT; ISSUE SUGGESTED/MANDATORY HEADING AND/OR ALT CHANGES; EVEN WHEN THE ACFT IS EQUIPPED WITH TCASII. 2) PLT: PARTICULARLY AT NIGHT; MAINTAIN KNOWN ALT SEPARATION FOR CONFLICTING ACFT; EVEN WHEN TCASII AND/OR VISUAL MOTION APPEARS TO SHOW MINIMALLY ACCEPTABLE SEPARATION. BE ESPECIALLY VIGILANT WHEN WORKLOAD (CAUSED IN PART BY ENHANCED AVIONICS) AND FATIGUE ARE DISTRACTIONS. DELAYING THE DSCNT UNTIL I HAD HAD MORE TIME TO EVALUATE THE SIT WOULD HAVE MADE THIS A 'NON-INCIDENT.'