B737 CREW HAD LESS THAN LEGAL SEPARATION WITH AN ACR AT MGGT.

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TFC CONFLICT ON APCH. WE CROSSED THE RABINAL VOR 12000 FT/250 KTS AND TURNED TOWARD THE AURORA VOR AND INTERCEPTED THE RADIAL INBOUND. WE WERE CLRED TO 11000 FT AND GIVEN THE ILS DME-2 RWY 1. THEN APCH TOLD US THAT WE WOULD BE #2 ON THE APCH AND ACFT Y WAS INBOUND ABOVE US; APPROX 2000 FT ABOVE; AND DSNDING OVER THE VOR. WE COULD SEE THE TFC AND WE WERE CLRED TO 9000 FT. MY FO WAS FLYING AND STARTED TO SLOW; SEEING THAT THE SPACING WAS LOOKING TIGHT. WE CROSSED THE AUR VOR AND HEADED OUT ON THE 215 DEG RADIAL. WE CONTINUED TO SLOW; 150 KTS; AND CONFIGURE BECAUSE THE TFC WAS STILL TOO CLOSE. ACFT Y WAS STILL VISUAL AT THIS POINT AND WE COULD SEE HIM START HIS L TURN BACK TOWARD THE ARPT ON THE PROC. WE LOST SIGHT OF HIM IN THE CLOUDS AND ESTIMATED HE WAS 4-5 MI IN FRONT OF US BY THIS TIME. APCH CLRED US FOR THE APCH AS WE STARTED THE PROC TURN AT 6.2 DME. HE TOLD US TO CONTACT THE TWR. AT APPROX THE 90 DEG POINT IN OUR TURN AND 6.8 DME WE SAW A TCASII TARGET COMING FROM THE R AND CONVERGING ON US. WE WERE DSNDING FROM 6600 FT TO 6300 FT AND HE WAS DSNDING TOWARD US. WE GOT A RESOLUTION ON TCASII; 'MONITOR VERT SPD;' THEN FO SAW THE TFC DOWN TO HIS R AND NOW WAS CLBING. HE MUST HAVE GOTTEN A TCASII TO CLB. HE PASSED ABOVE US AND WE FLEW THROUGH THE LOC AND TURNED BACK TO INTERCEPT. WE WERE CLRED TO LAND BY THE TWR WITHOUT FURTHER INCIDENT. ON THE GND; I CALLED THE TWR TO ASK WHAT HAPPENED OUT THERE WITH ACFT Y. HE SAID ACFT Y WAS CLRED FOR THE APCH AND THAT HE WAS MONITORING US IN THE TURN; BUT DID NOT SAY ANYTHING BECAUSE HE THOUGHT WE COULD SEE EACH OTHER. I REMINDED HIM THAT WE WERE CLRED FOR THE ILS/DME APCH AND NOT A VISUAL APCH SINCE IT WAS INTERMITTENT IFR. I TOLD HIM THAT WE WERE AT 6.8 DME ON THE PROC AND FOR ACFT Y TO BE APCHING FROM OUR R ON THE FINAL APCH SEGMENT THAT HE MUST HAVE BEEN OUTSIDE THE 7.0 DME ARC. HE SAID THAT HE WAS GOING TO DISCUSS THE SIT WITH APCH. I TOLD HIM THAT I WAS GOING TO FILE A RPT ON THE INCIDENT. ACFT Y FINALLY LANDED AND THE CAPT CAME OVER TO OUR GATE AND I ASKED WHAT DME FIX HE USED AND HE SAID 7 DME. I BELIEVE HE MIGHT HAVE STARTED OUT AT 7 DME BUT HE MUST HAVE GONE BEYOND THAT IN HIS TURN TOWARD THE FINAL APCH SEGMENT. WE HAD NO SPD OR SPACING INSTRUCTIONS FROM APCH. WE DID OUR OWN BASED ON VISUAL WITH ACFT Y. I FEEL THAT APCH SHOULD NOT HAVE CLRED US FOR THE APCH UNTIL THE ACFT WAS ON THE FINAL APCH SEGMENT AND CLOSE TO THE FINAL APCH FIX BEFORE CLRING ANOTHER ACFT FOR THE APCH. APCH APPARENTLY TURNED ACFT Y OVER TO THE TWR AND NEVER MONITORED HIS POS AFTER THAT. I RECOMMEND THAT THE GUA APCH FACILITY CHANGE THEIR SOP AND MONITOR ALL ACFT TO THE FINAL APCH FIX TO INSURE EVERY ACFT HAS THE PROPER SPACING BEFORE TURNING THEM OVER TO THE TWR FREQ.