B757 WAS DSNDED ON TOP OF ANOTHER ACFT.

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SNA TO DFW; JUL/XA/99. CAPT (ME) FLYING. AT ABOUT XA30Z; WE WERE ON THE BOWIE 5 ARR JUST PAST BAMBE. WE WERE ON A VECTOR AND HAD BEEN CLRED DOWN TO 7000 FT. AS WE APCHED THAT ALT; WE NOTICED ON TCASII AN ACFT 6000 FT AND AT OUR 9 O'CLOCK POS AT ABOUT 2 MI. WE SAW VISUALLY THAT IT WAS A TURBOPROP; SAME GENERAL DIRECTION AT A SLIGHT XING ANGLE; APPARENTLY SLIGHTLY SLOWER. AS WE WERE ALMOST LEVEL; APCH CTL SAID 'DSND TO 6000 FT.' WE STARTED DOWN; WATCHING THE TCASII RETURN; WHICH BY THEN SHOWED JUST ABOUT DIRECTLY BELOW US. WE WERE EXPECTING THAT HE ALSO WOULD BE DSNDING OR TURNING AWAY. HOWEVER; HE DID NOT. WE GOT TO ABOUT 6600 FT BEFORE I CLICKED OFF THE AUTOPLT AND SAID TO FO; 'ASK HIM ABOUT THAT ALT.' JUST THEN; APCH CTLR SAID 'I TOLD YOU TO MAINTAIN 7000 FT.' I BELIEVE WE SAID NO; WE LEVELED AT 7000 FT AND THEN WERE CLRED TO 6000 FT. CTLR SAID IT DIDN'T MATTER; AND WHEN WE ASKED IF HE WANTED US TO CLB UP TO 7000 FT HE SAID NO; DSND TO 5000 FT. IT'S ALWAYS POSSIBLE THAT WE MISHEARD HIM; OR THAT MY RECOLLECTION OF THE BRIEF SUBSEQUENT EXCHANGE IS NOT CORRECT; BUT BOTH OF US IN THE COCKPIT ARE QUITE POSITIVE THAT WE WERE CLRED FIRST TO 7000 FT; THEN TO 6000 FT. NO TFC OR CONFLICT WAS EVER CALLED OUT TO US. BUT THE POTENTIAL FOR DISASTER IS QUITE OBVIOUS. SINCE WE WERE DIRECTLY ON TOP OF THE OTHER ACFT; NEITHER OF US WOULD HAVE SEEN THE OTHER IF WE HAD CONTINUED DSCNT INTO HIM. SINCE WE WERE WATCHING EVENTS CLOSELY ON TCASII; I WOULD NOT CALL IT A NEAR MIDAIR; BUT IF WE HAD NOT BOTH BEEN TCASII EQUIPPED; IT CERTAINLY WOULD HAVE BEEN. SUPPLEMENTAL INFO FROM ACN 444592: WE WERE ON THE UKW 5 ARR AND PUT ON RADAR VECTORS E OF BAMBE. ON A VECTOR WE WERE CLRED FROM 11000 FT TO 7000 FT. AT APPROX 7000 FT ON DSCNT WE PICKED UP AN ACFT ON TCASII APPROX 1000 FT BELOW AT 9 O'CLOCK; 2 MI. THE CAPT VISUALLY ACQUIRED A TURBOPROP WITH A SLIGHT XING ANGLE. APCHING 7000 FT; WE WERE CLRED TO 6000 FT. WE CONTINUED OUR DSCNT WATCHING THE ACFT ON TCASII. IT WAS APPARENT THE XING ANGLE WAS GOING TO PUT HIM UNDER US SO THE CAPT DISCONNECTED THE AUTOPLT AND LEVELED OFF AT 6500 FT. THE CAPT ASKED ME TO QUERY APCH ON BOTH THE ALT CLRNC AND TFC. AT THAT TIME APCH SPOKE UP AND SAID WE WERE CLRED TO 7000 FT. I SAID WE WERE CLRED TO 6000 FT BUT WE LEVELED AT 6500 FT BECAUSE OF TFC BELOW US. I ASKED IF HE WANTED US TO CLB TO 7000 FT. HE SAID; NO AND GAVE US A VECTOR AND FARTHER DSCNT TO 5000 FT. WE WERE BY THEN CLR OF TFC. WE NEVER GOT AN RA. COCKPIT VIGILANCE PREVENTED A NEAR MISS.