ACR X HAD NMAC LTSS FROM ACR Y. EVASIVE ACTION TAKEN. SEE AND AVOID. SYS ERROR.

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BOTH ACFT WERE IFR INBOUNDS TO BWI. ON FREQ 119.7 ACR X HAD BEEN GIVEN SEVERAL VECTORS FOR SEQUENCING TO VISUAL APCH FOR RWY 33R. ON A HDG OF 040 DEG AT 4000 FT MSL WE WERE GIVEN TO FREQ 119.0. WE WERE ON THIS FREQ FOR APPROX 1 MIN 30 SECONDS WITHOUT BEING ABLE TO ESTABLISH CONTACT DUE TO FREQ CONGESTION. THE FO WAS HAND FLYING AND LOOKING TOWARD THE L AT THE ARPT. I WAS SCANNING BACK AND FORTH AND HAD JUST SEEN AN SMA PASS BELOW US GOING IN THE OPPOSITE DIRECTION THAT HAD NOT BEEN CALLED. I TURNED TO TELL THE FO 'THIS GUY IS BUSY;' BUT ALL OF A SUDDEN I SAW ACR Y IN THE FO'S WINDSCREEN MOVING R TO L AT THE SAME ALT -- APPROX 1/4 TO 1/2 MI IN FRONT OF US. I IMMEDIATELY GRABBED THE CTL WHEEL AND PITCHED DOWN WITHOUT SAYING A WORD TO THE FO. WE PASSED LESS THAN 100 FT APART AND WENT THROUGH THE ACR Y WAKE AND PROP WASH. THE ACR Y MUST HAVE SEEN US ABOUT THE SAME MOMENT I SAW HIM AND HE BEGAN A PITCH UP MANEUVER. ON THE GND I SPOKE WITH THE CAPT OF ACR Y. HE SAID HIS FO WAS FLYING AND HE (THE CAPT) SAW ME FIRST AND PITCHED UP. (NEITHER ACFT HAD TCASII). HE HAD BEEN ON FREQ 124.55 AND HAD BEEN SWITCHED TO 119.0 ON A HDG OF 280 DEG AT 3000 FT. HE TOO HAD BEEN UNABLE TO ESTABLISH CONTACT DUE TO FREQ CONGESTION. WE BOTH SPOKE TO THE APCH CTL SUPVR VIA LANDLINE. HE ADMITTED TO ME THAT THERE HAD BEEN A CTLR PROCEDURAL ERROR AND HE WAS 'REVIEWING.' NEITHER ONE OF US HAD A CHANCE TO TALK ON 119.0 AND FOR THAT CTLR TO RESOLVE THE CONFLICT; IF HE HAD SEEN IT; TIME WAS MINIMAL; CONSIDERING HIS WORKLOAD AT THE MOMENT. THIS WAS A VERY CLOSE INCIDENT AND TRAUMATIC FOR THE PAXS IN ACR Y WHO COULD SEE IT COMING BEFORE EITHER ACFT'S PLTS. BOTH PLTS STATED THE NEAR MISS TO THE CTLR ON FREQ 119.0. HIS COMMENT WAS 'I WASN'T TALKING TO EITHER OF YOU.' HE TOLD US TO FILE OUR RPTS AS NECESSARY. SUPPLEMENTAL INFO FROM ACN 212896. SECTOR WAS EXTREMELY BUSY. ACR X WAS ON A 040 HDG; SEQUENCE FOR RWY 33R AT 3000 FT. ACR Y ON A 280 DEG HDG AT 3000 FT ALSO FOR 33R. ACFT CAME FROM DIFFERENT SECTORS. PLT OF ACR Y HAD NOT CHKED ON THE FREQ. ACR X; CHKED ON; AND I DSNDED ACR X TO 2000 FT; HOWEVER THE NMAC OCCURRED SIMULTANEOUSLY. I BELIEVE THIS INCIDENT WAS CAUSED BY TOO HVY A VOLUME IN THE SECTOR; AND NOT TALKING TO EITHER PLT UNTIL IMMEDIATELY BEFORE THE INCIDENT. PLT OF ACR Y SAID AFTER THE INCIDENT; THAT HE HAD TRIED TO CALL BUT THE FREQ WAS TOO BUSY.