ACR X CLB THROUGH OCCUPIED ALT HAD LTSS FROM SMT Y. SYS ERROR. EVASIVE ACTION TAKEN BY SMT Y.

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ALLENTOWN DEP ASSIGNED ACR X A 240 DEG HDG AND CLRED US TO 6000 FT (WE WERE LEVEL AT 4000 FT). PASSING THROUGH 4500 FT DEP ADVISED TFC AT 1 O'CLOCK SEBOUND 3 MI AT 5000 FT. DEP THEN ADVISED TURN L HDG 180 (NOTHING ELSE WAS SAID). AS I WAS IN THE TURN THE FO SAID TFC IS ON A COLLISION COURSE WITH US. I TRIED LOOKING FOR THE TFC BUT DIDN'T HAVE A VISUAL. THE TFC (WHICH WAS A SMT Y) DID AN IMMEDIATE L TURN TO AVOID US. THE FO ASKED DEP IF THEY SAW WHAT HAPPENED. THERE ONLY COMMENT WAS; 'YES.' THERE WAS NO OTHER TRANSMISSIONS CONCERNING THE INCIDENT. I HAD NO TIME TO MANEUVER. I'M JUST GLAD THE OTHER GUY SAW US. THE CTLR SEEMED VERY PASSIVE ABOUT THE WHOLE INCIDENT. WHEN HE TOLD US TO TURN TO 180 DEGS HIS VOICE WAS NOT THAT OF EMER OR DIDN'T EVEN SAY 'TURN IMMEDIATELY.' A CONVERSATION BTWN THE FO AND SUPVR FROM ALLENTOWN ATC REVEALED AFTER REVIEWING THE TAPE THAT OUR CTLR WAS WORKING 2 ACFT BESIDES US. A SMT AND THE SMT Y. BOTH WERE HDG ESE AT 5000 FT. 8 MI IN TRAIL TO EACH OTHER. WHEN THE FIRST SMT WAS BY US; THE CTLR WHO TOLD HIM TO MAKE THE L TURN TO AVOID US. ALSO; ZNY HAD THE INCIDENT ON TAPE. VERT SEPARATION - 0; HORIZ - 600 FT.