ACR MLG THAT STRAYED OFF THE FINAL APCH COURSE WHILE ON PARALLEL ILS APCH. CAUSES CONFLICT.

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ON A 330 DEG HDG AT 6000' TO INTERCEPT RWY 30L LOC AT STL; XING THE MISSISSIPPI RIVER S OF THE GATEWAY ARCH WE WERE ADVISED OF MLG Y AT 2:30 TO 3 O'CLOCK ON A PARALLEL APCH TO 30R. HE WAS 1000' BELOW US. TFC WAS SIGHTED AND APCH CTL WAS SO ADVISED. FLT VISIBILITY WAS HAZY AND ABOUT 3 TO 5 MI WITH GND CONTACT BUT THE ARPT WAS NOT IN SIGHT. WE WERE CLRED FOR 30L ILS TO MAINTAIN 2200' OR HIGHER UNTIL ESTABLISHED ON THE LOC AND TO MAINTAIN 190 KTS TO THE OUTER MARKER. WE VISUALLY SIGHTED THE RWY IN THE DSCNT AND TURNED SO AS NOT TO INTERCEPT THE LOC BUT TO REMAIN SLIGHTLY SOUTH OF THE RWY CENTERLINE. REALIZING THAT THE PARALLEL RWYS ARE CLOSE TOGETHER; THE CAPT ELECTED TO WAIT TO CENTER THE LOC UNTIL HE RECONFIRMED THE POS OF MLG Y THAT WAS MAKING THE PARALLEL APCH. AT THE SAME TIME THAT I WAS GOING TO ALERT THE CAPT THAT MLG Y WAS GETTING TOO CLOSE HE LOOKED OUT THE SIDE WINDOW AND SAW MLG Y CORRECTING BACK TO THE 30R LOC AT A DISTANCE OF APPROX 100' ABOVE AND 500' HORIZONTALLY FROM US IN A 20 TO 30 DEG BANK TO THE RIGHT. THIS INCIDENT COULD HAVE BEEN AVOIDED IF THE INTERCEPTS TO THE PARALLEL LOCS HAD BEEN STAGGERED; I.E. NOT WING TIP TO WING TIP. FOR AN ARPT LIKE STL-LAMBERT WHERE PARALLEL RWYS ARE TOO CLOSE TOGETHER; SIMULTANEOUS APCHS SHOULD ONLY BE MADE WITH STAGGERED INTERCEPTS. IT IS MUCH EASIER AND SAFER TO VISUALLY MAINTAIN CONTACT WITH YOUR TFC IF IT IS IN FRONT OF YOU VS ON YOUR WING AT 9 OR 3 O'CLOCK.