A320 FLC INVOLVED IN AN NMAC AFTER ACCEPTING AN ENRTE VISUAL SEPARATION CLB WITH OPPOSITE DIRECTION TFC.

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DEPARTING DTW FOR ATL BTWN CAVVS INTXN AND VWV AT 17000 FT. CONVERSATION WENT LIKE THIS: ATC: ACR X; TFC 11 O'CLOCK POS; 10 MI; FL180 HIGHER WHEN CLR. ACR X: 'ROGER; TFC IN SIGHT.' ATC: 'ACR Y; TFC 11 O'CLOCK; POS; 10 MI; 17000 FT.' ACR Y: 'ROGER; TFC IN SIGHT.' ATC: ACR Y; MAINTAIN VISUAL SEPARATION; HE'S GOING TO CLB THROUGH YOUR ALT.' I DON'T KNOW IF ACR Y RESPONDED; OR NOT. ATC STATED: 'ACR X; ACR Y'S GOING TO MAINTAIN VISUAL SEPARATION; CLB; AND MAINTAIN FL230.' ACR X: 'ROGER; FL230.' AND WE (ACR X) STARTED CLBING. TCASII TFC; RA ALARMED; WHICH I EXPECTED SINCE LATERAL SEPARATION WAS LIKELY LESS THAN TCASII TOLERANCE; WHATEVER THAT IS. TCASII RA COMMANDED DSND. NOW ACR Y WAS CLOSE ENOUGH TO SEE THAT HE WAS IN A L TURN TOWARDS US. I DISENGAGED AUTOPLT AND LEVELED AS ACR Y PASSED VERY CLOSE ABOVE; VERY; VERY CLOSE. HE DID NOT PASS DIRECTLY OVER THE COCKPIT; BUT I'M CONFIDENT AT LEAST PART OF HIS ACFT WAS DIRECTLY OVER PART OF MINE. I AM CERTAIN VERT SEPARATION WAS UNDER 500 FT. VISUAL SEPARATION ENRTE AT NIGHT IS AN ACCEPTABLE PRACTICE TO THE FAA; BUT IT SHOULDN'T BE. VERY FEW PLTS HAVE ANY NIGHT FORMATION TRAINING AND DEVIATING FROM YOUR IFR RTE CLRNC TO MAINTAIN SEPARATION IS CONTRARY TO ALL OUR HABIT PATTERNS. I NEVER SHOULD HAVE BEGUN THAT CLB. THE ALPHA POS IS AGAINST THIS PROC AND I SHOULD HAVE FOLLOWED THEIR ADVICE.