LATE CLRNC CAUSED ALTDEV DSCNT THROUGH ASSIGNED ALT BY ACR X TCASII RA IGNORED WITH ACR Y.

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AFTER CHKING IN WITH ONT APCH; ACR X GIVEN DSND CLRNC FROM 15000 FT DOWN TO 10000 FT. WE WERE FLYING THE KAYOH 3 ARR INTO SNA. WE WERE PROCEEDING DIRECT TO KAYOH INTXN AS INSTRUCTED BY ONT APCH (134.00) OUR FMC PROFILE SHOWED US TO BE APPROX 2100 FT HIGH FOR THE ANTICIPATED RESTR AT KAYOH INTXN OF 210 KTS AND 8000 FT AS NORMAL ISSUED BY ONT APCH WE WERE DSNDING AT APPROX 3500 FPM DSNDING THROUGH 12400 FT WHEN ONT APCH AMENDED OUR ALT TO 12000 FT. THE FO DISCONNECTED THE AUTOPLT TO TRY TO STOP OUR RATE OF DSCNT AS QUICKLY AS POSSIBLE TO LEVEL AT 12000 FT. AT ABOUT 12100 FT WE RECEIVED A TCASII ADVISORY OF 'TFC; TFC.' WE OBSERVED AN AMBER FILLED CIRCLE ON OUR VSI-TCASII DISPLAY CLBING AT OUR 1-2 O'CLOCK POS READING R TO L. AT APPROX 11600 FT WE RECEIVED A DSND; DSND; DSND RA FROM TCASII; BY NOW THE FO IS CLBING BACK TO 12000 FT. I HAD VISUAL CONTACT WITH THE TFC NOW AT OUR 12 O'CLOCK POS: (A B-737-300). I INSTRUCTED MY FO TO IGNORE THE RA AND MAINTAIN 12000 FT. ONT APCH WAS SLOW IN PROVIDING IFR SEPARATION WITH MY FLT; AS WELL AS THE ACR Y FLT. WE WERE NEVER ADVISED OF THE Y FLT XING BELOW US.