APPARENT LOSS OF SEPARATION BTWN RPTR'S B757 ON BASE TO FINAL FOR THE CTR RWY AND AN ACR MD80 CONVERGING XING COURSE BELOW AND FROM THE R FOR THE L RWY.

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COMING FROM DEN TO DFW ON VISUAL APCH L BASE RWY 35C. CLRED FOR VISUAL APCH WAS AROUND 9000 FT AGL. WE STARTED OUR TURN TOWARDS FINAL WHEN I NOTICED AN S80 ON A CONVERGING APCH LOWER TO THE R. I QUERIED APCH ABOUT THE SUPER 80 AND WHERE IT WAS GOING. THE CTLR TOLD US TO MAINTAIN 4000 FT. WE ACKNOWLEDGED AND THE CTLR ASKED IF WE HAD VISUAL ON TFC. I AFFIRMED. I TOLD THE CAPT TO STOP DSCNT DUE TO TFC ABOUT THE TIME THE TCASII RA CONCURRED MY STOP DSCNT. WE WERE NOT APPRISED OF THE SUPER 80 TFC AND THE SEE AND BE SEEN VISUALLY IS WHAT KEPT US FROM THE SUPER 80. THE CAPT SAID THE RA SHOWED ONLY 600 FT SEPARATION. I WAS UNABLE TO CONFIRM THIS DUE TO MY EYES NOT LEAVING THE SUPER 80. WHEN CONDUCTING THE XOVER TFC IT'S IMPERATIVE TFC CALLS ARE RELAYED. SUPPLEMENTAL INFO FROM ACN 387080: WHILE ON L DOWNWIND TO LAND N AT DFW RWY ASSIGNMENT WAS TO BE RWY 35C -- UNUSUAL. TFC WAS AN S80 TURNING FINAL TO WHICH RWY WE DID NOT KNOW. WE WERE THEN FOR THE VISUAL TO RWY 35C TO MAINTAIN 4000 FT TILL TURNING FINAL. APCH HAD SET US UP CLOSE AND HIGH. A TURN WAS MADE TO INTERCEPT FINAL AND THE DSCNT WAS BEGUN WHEN WE PICKED UP THE S80 TFC AND SHORTLY THEREAFTER AN RA WAS GIVEN FROM TCASII. WE COMPLIED AND QUERIED APCH CTL ABOUT THE TFC. FOR THE FIRST TIME WE LEARNED THAT HE WAS TO PASS UNDER US ON A R BASE FOR RWY 35L. UP UNTIL NOW NO MENTION WAS MADE ABOUT THE XOVER PLAN. IT SEEMED AN UNNECESSARY AND POTENTIALLY HAZARDOUS PLAN WITH NO HEADS UP AS TO WHAT WAS GOING ON.