MD80 AND B737; OPPOSITE DIRECTION; TAKE EVASIVE ACTION WHEN MD80 READS BACK INCORRECT DSCNT ALT; WHICH ZKC CTLRS DO NOT CATCH UNTIL BOTH ACFT INITIATE TCASII ACTION.

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DSNDED ACR X TO FL290. ACR Y WAS AT FL280. ACR X READ BACK THAT THEY WERE OUT OF FL330 FOR FL240. RADIOS WERE SCRATCHY; I MISSED THE READBACK. THE SECTOR WAS VERY COMPLEX AT THE TIME. I HAD NUMEROUS ACFT ON VECTORS FOR SPACING FOR ST LOUIS. BY THE TIME I WAS AWARE THAT ACR X HAD DSNDED INTO ACR Y'S ALT; THE ACFT HAD ALREADY RESPONDED TO TCASII. CONFLICT ALERT DID NOT ACTIVATE UNTIL AFTER THE ERROR OCCURRED. THE BEST WAY TO HELP CTLRS WITH THESE TYPES OF ERRORS; WOULD BE TO MAKE PLTS ACCOUNTABLE FOR LISTENING TO CTL INSTRUCTIONS. IT WOULD GIVE THEM MORE INCENTIVE TO PAY ATTN -- BE A TEAM PLAYER. SUPPLEMENTAL INFO FROM ACN 460210: ACFT #2 WAS WBOUND AT FL280. ACFT #1 EBOUND ON TRAKE 8 STAR WAS DSNDED FROM FL330 TO FL290. PLT READ BACK 'FL240' AND DSNDED INTO ACFT #2'S ALT. BOTH ACFT RESPONDED TO RA'S. SUPPLEMENTAL INFO FROM ACN 469760: I WAS THE FO (PF) ON ACR X FROM PHX TO STL. WHEN WE WERE APPROX 100 MI W OF BUM AT FL330; ZKC CLRED US TO FL240. AT AROUND FL265; WE RECEIVED A TCASII RA INSTRUCTING US TO CLB FOR AN ACFT AHEAD AND 800-1000 FT BELOW US. WE CLBED BACK UP TO ABOUT FL280. ZKC ASKED US WHY WE DID NOT LEVEL AT FL290. THE CAPT ADVISED THEM WE WERE CLRED TO FL240.