ACR REACTING TO TCASII ON A VFR ACFT ABOVE; DSNDS TOWARDS AN IFR ACFT BELOW.

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ACR X WAS CONTACTING ME FROM THE FEEDER CTLR. THERE WAS A VFR ACFT AT 10500 MSL; ACFT Z; IN SAME LOCATION. ALSO; THERE WAS ACR Y; AN LTT; DIRECTLY BELOW HIM DSNDING OUT OF 8000 MSL. ACR X WAS LEVEL AT 10000. ACR X CHKED IN ON THE FREQ DECLARING THAT HE WAS RESPONDING TO AN RA. HE ADVISED HE HAD DSNDED AND THAT HE WAS RETURNING TO HIS ASSIGNED ALT. THIS WAS A PARTICULARLY DANGEROUS SIT. ACR X APPARENTLY HAD MANEUVERED WHEN HE WAS BTWN FREQS. HAD A CTL ACTION BEEN NECESSARY; IT WOULD NOT HAVE BEEN POSSIBLE. ADDITIONALLY; BECAUSE THE ARTS TAGS HAD MERGED; THE CURRENT ALT DATA WAS UNAVAILABLE TO ME. I; BY CHANCE; NOTICED ACR Y WAS LEAVING 8000 JUST BEFORE ACR X CHKED IN AND THE TAGS MERGED. (ALSO; ACR Y WAS ON ANOTHER CTLR FREQ THAT WAS VECTORING HIM TO A DIFFERENT RWY. DURING THIS SIT AT ORD; ACR Y WAS ON ANOTHER CTL SYMBOL. ACR Y'S ORIGINAL ASSIGNED ALT WAS 9000 FT MSL. ADDITIONALLY; THE FLT PROGRESS STRIP ON ACR X HAD A '10' ON IT. THIS ADVISED ME TO NOT DSND BELOW 10000 UNTIL FURTHER ADVISED BY MY HDOF MAN.) I FEEL THAT THE TCASII MANEUVER TO MISS A VFR ACFT (WHICH WAS STANDARD SEPARATION ABOVE HIM) WAS UNNECESSARY AND CREATED UNDUE STRESS ON MYSELF. TCASII IS NOT AWARE OF VFR TFC VS IFR TFC AND WOULD CREATE A 'SPLIT THE DIFFERENCE' SIT GIVEN A VFR 500 FT ABOVE AND AN IFR 1000 FT BELOW. TCASII WAS SUPPOSED TO ENHANCE SAFETY; NOT JEOPARDIZE IT.