PIC OF ACR LGT COMPLAINT ABOUT HAVING TO MAINTAIN VISUAL SEPARATION WITH 2 ACFT WHILE ON APCH.

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WE WERE FOLLOWING A WDB ON A VISUAL APCH; ABOUT 6 MI IN TRAIL; ON FINAL TO DFW RWY 36L. THE APCH CTLR ASKED IF WE HAD AN SMT IN SIGHT AT 2 O'CLOCK; 3 MI. THE F/O SAW HIM AND SAID YES. I DID NOT SEE HIM. THE CTLR SAID; 'KEEP AN EYE ON HIM; HE IS VISUAL TO 36R; CONTACT THE TWR.' (I PRESUMED 'KEEP AN EYE ON HIM' MEANT MAINTAIN VISUAL SEP.) I CONTINUED FOLLOWING THE WDB WITH BRIEF SEARCHES TO THE RIGHT LOOKING FOR THE SMT. THE F/O LOST SIGHT OF HIM. WE ASKED THE TWR CTLR TO POINT HIM OUT. I DID SEE HIM WITH 1 OF MY LOOKS TO THE RIGHT. ONE PROB WAS WE WERE HIGHER AND LOOKING DOWN; THE SMT WAS CAMOUFLAGED BY THE GND AND BLDGS. THIS WAS THE FIRST TIME I REMEMBER SHOOTING A VISUAL APCH WITH THE ASSIGNMENT TO MAINTAIN VISUAL SEP WITH 2 ACFT. THE TASK WAS LITERALLY IMPOSSIBLE. I CALLED THE APCH CTL FAC AND DISCUSSED THE SITUATIONS WITH A CTLR. HE SAID THERE WAS NO LIMIT TO THE NUMBER OF ACFT WITH WHICH A PLT OR CTLR CAN MAINTAIN VISUAL SEP. ANOTHER INTERESTING POINT: RWY 36L IS ABOUT 1200' FROM RWY 36R. THE SMT WAS ABOUT 3 MI BEHIND AND TO THE RIGHT OF A WDB WITH THE WIND FROM 310 DEGS AT 12 KTS. THE SMT WAS IN A DANGEROUS PHASE. THIS MAY BE LEGAL; BUT IT IS NOT AS SAFE AS IT SHOULD BE OR COULD BE!!