ACR X TCASII RA DSND WITH VFR ACFT HAD LTSS FROM ACR Y. PLTDEV. EVASIVE ACTION TAKEN.

Narrative

ACR X DC-9 AT 12000 FT. ACR Y BA31 AT 11000 FT. UNKNOWN VFR AT 12400 FT. X WAS OVERTAKING Y; BOTH VECTORED ON A 050 DEG HDG. AN UNKNOWN VFR ACFT WAS SW BOUND INDICATING 12400 FT. I TURNED Y 15 DEG TO THE L TO GET SPACING FOR THE FINAL CTLR. 14 SECONDS LATER; X ADVISED HE WAS DSNDING IN RESPONSE TO A RA; TO THE TFC AT 12400 FT. THE FAA'S INSTRUCTION TO CTLRS IS TO ACKNOWLEDGE THE DEV BUT NOT TO CONTRADICT IT. I ROGERED THE ANNOUNCED DSCNT AND ADVISED X THAT Y WAS ALSO BELOW HIM AT 11000 FT. I ASSUMED HE WOULD ONLY DSND 200 FT BUT ACCORDING TO THE RADAR DISPLAY HE WENT DOWN TO 11000 FT THEN CLBED BACK. THE RESULTING INITIAL READOUT INDICATED SEPARATION OF 1.2 MI LATERALLY AND 0 FT VERTLY. HAD I NOT TURNED Y EARLIER; THE SEPARATION WOULD HAVE BEEN EVEN LESS. THIS IS THE SIT THAT ALL CTLRS HAVE WONDERED WHEN IT WOULD HAPPEN. AN UNCTLED VFR AT A LEGAL ALT GIVE AN IFR ACFT A DSND (OR CLB) RESOLUTION INTO ANOTHER IFR ACFT. TCASII WORKS WITH ONE SIT ONLY AT A TIME (KIND OF LIKE TUNNEL VISION); IGNORING OTHER ACFT. A MAJORITY OF THE CTLRS AT MY FACILITY FEEL THAT TCASII SHOULD ONLY BE USED IN AN ADVISORY MODE IN THE TERMINAL ENVIRONMENT.