DEP CTLR CLBING 2 ACR MD80'S ON WESTERLY VECTORS; WITH THE SECOND MD80 MAINTAINING VISUAL SEPARATION FROM THE FIRST MD80. THE FIRST MD80 DSNDED TO WITHIN 100 FT OF THE SECOND ACFT WHILE RESPONDING TO A TCASII ALERT. NMAC OCCURRED EVEN THOUGH THE FIRST ACFT WAS ADVISED OF THE VISUAL SEPARATION IN USE.

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ACFT #1 DEPARTED RWY 17R AT DFW AND APPROX 6 MI S OF THE ARPT WAS ASSIGNED A WBOUND HDG. APPROX 3 MI IN TRAIL OF ACFT #1; ACFT #2 DEPARTED DFW SBOUND. ACFT #2 RPTED ACFT #1 IN SIGHT AND WAS INSTRUCTED TO MAINTAIN VISUAL SEPARATION; MAINTAIN 9000 FT; AND TO TURN R HDG 270 DEGS. ACFT #1 WAS ADVISED OF ACFT #2'S VISUAL SEPARATION AND WAS CLBING FROM 8500 FT TO 17000 FT. WHEN ACFT #2 LEVELED AT 9000 FT; ACFT #1 WAS OBSERVED CLBING FROM 9500 FT. ACFT #1 THEN ADVISED THAT HE WAS FOLLOWING A TCASII RA AND DSNDING FROM 9500 FT. ACFT #1 DSNDED TO 9100 FT OVER ACFT #2 AT 9000 FT. TO PREVENT RECURRENCE: TCASII IS AN EXCELLENT TOOL WHEN UTILIZED APPROPRIATELY. THE CONFLICT COMES WHEN DEFINING 'APPROPRIATE UTILIZATION.' IT APPEARS LOGICAL TO MOST CTLRS THAT WHEN ATC INSTRUCTIONS ARE ISSUED (AS IN THIS SIT) TO PREVENT A COLLISION; PLTS SHOULD FOLLOW ASSIGNED INSTRUCTIONS RATHER THAN TCASII RA'S. I ALSO UNDERSTAND THE DESIGN OF TCASII TO PROVIDE SAFETY WHEN ATC INSTRUCTIONS ARE INADEQUATE. THIS SIT CONFIRMS THE THEORY THAT TCASII RA'S MUST BE A 'SECOND' LINE OF DEFENSE AND SHOULD NOT BE ALLOWED TO TAKE PRECEDENCE OVER ATC INSTRUCTIONS.