AFTER ACCEPTING VISUAL SEPARATION; FA20 FLC CONSIDERS OPTIONS SEEING VERT AND LATERAL DISTANCES DECREASE FROM THE OTHER ACFT; TAKING ACTION ONLY WHEN TCASII ALERTS.

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THIS WAS THE DSCNT PHASE OF OUR FLT HDG SSE FOR A LNDG AT CDN. WE WERE WORKING CLT APCH AND HAD BEEN ASSIGNED 14000 FT BECAUSE OF ANOTHER ACR AT 10 O'CLOCK POS; 13000 FT; 15 NM; THAT WAS ALSO DSNDING FOR A LNDG AT CLT. OUR COURSES WERE CONVERGING. WE ESTABLISHED VISUAL CONTACT WITH THE OTHER ACFT AND RPTED THIS TO CLT APCH; WHO THEN CLRED US TO DSND TO 11000 FT WITH VISUAL REF TO THE OTHER ACFT. CLT ALSO ADVISED THE OTHER ACFT OF OUR DSCNT WHICH WAS ACKNOWLEDGED BY THEM. THE CAPT DSNDED QUICKLY BELOW THE OTHER ACFT TO 11000 FT AND LEVELED THERE. I DO NOT KNOW WHETHER THE OTHER ACFT WAS CLRED TO CONTINUE ITS DSCNT; BUT I OBSERVED ON THE TCASII THAT IT WAS CONTINUING TO DSND THROUGH 12000 FT; AND BY NOW OUR HORIZ SEPARATION WAS 7 MI ON A CONVERGING COURSE. I ADVISED THE CAPT THAT WE NEEDED TO TAKE ACTION; HOWEVER; HE SEEMED UNSURE OF JUST WHAT TO DO. I OBSERVED THAT THE OTHER ACFT HAD STOPPED THE DSCNT APPROX 500 FT ABOVE US. THE TCASII SOUNDED A TA; FOLLOWED BY AN RA TO DSND. AT THIS POINT; THE TCASII SHOWED 500 FT VERT; AND 4 MI HORIZ SEPARATION. THE CAPT DISCONNECTED THE AUTOPLT TO COMPLY WITH THE RA. AS I WAS REACHING FOR THE MIKE TO RPT A TCASII DSCNT; CLT RECLRED US TO DSND TO 3000 FT; WHICH WAS EXECUTED IMMEDIATELY. THE OTHER ACFT PASSED OVERHEAD AT APPROX 1000 FT. THIS SIT PRESENTED SOME PRACTICAL DILEMMAS; WHICH CONTRIBUTED TO THE DIFFICULTY IN MAKING A TIMELY DECISION: ('BOXED IN'). 1) TURNING L WOULD ONLY INCREASE OUR CLOSURE RATE; AGGRAVATING THE SIT. (OTHER ACFT AT 10 O'CLOCK POS; 7 M8.) 2) TURNING R WOULD PUT THE OTHER ACFT OUT OF OUR SIGHT; COMPROMISING REF BASED ON VISUAL SEPARATION. 3) THE OTHER ACFT WAS ABOVE US; SO CLBING WOULD ONLY DECREASE SEPARATION. 4) WE HAD DSNDED TO OUR CLRED ALT (11000 FT) SO A DSCNT WAS NOT TECHNICALLY ALLOWED UNTIL THE TCASII RA SOUNDED. 5) THE OTHER ACFT HAD STOPPED ITS DSCNT. I HAVE LONG FELT THAT IFR CLRNCS BASED ON VISUAL SEPARATION AT NIGHT ARE NOT WISE. IT IS MORE DIFFICULT TO JUDGE ONE'S RELATIVE POS TO OTHER ACFT AT NIGHT. AND THOUGH NOT THE CASE IN THIS INSTANCE; THE CHANCES OF FIXING ON THE WRONG ACFT INCREASE AT NIGHT. THIS INCIDENT FURTHER CONFIRMS MY BELIEF THAT MANEUVERS BASED ON VISUAL SEPARATION SHOULD NOT BE PERMITTED AT NIGHT FOR ACFT UNDER IFR CTL. CLT APCH DID NOT INDICATE BEFORE HAND OR COMMENT AFTER; BUT I BELIEVE THEY WERE EXPECTING US TO DSND VISUALLY ABOVE THE OTHER ACFT.