ATX X AND Y NON ADHERENCE TO ATC INSTRUCTION UNAUTH XPONDER CODE SWAP CAUSING DATA TAG SWAP. PLTDEV.

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DURING MY SHIFT I WAS WORKING 6 COMBINED SECTORS AT ZSE. AT APPROX XX00; ATX X; OAK-PDX; CHKED ON FREQ S OF FJS VORTAC AND ATX Y; PDX-OAK; CHKED ON AT EUG; BOTH ACFT WERE OPERATING VFR ON TOP. THE ACFT WERE ON OPPOSITE COURSES AND PASSED DIRECTLY OVER EACH OTHER. THEY WERE TALKING TO EACH OTHER ON COMPANY FREQ. WHEN THE ACFT PASSED EACH OTHER THE FLT INTENTIONALLY CHANGED THEIR BEACON CODES TO THE OTHER ACFT'S SQUAWK. MAYBE IT WAS THE HR OF THE DAY; BUT THE DATA BLOCK SWAP WAS NOT NOTICED IMMEDIATELY. AFTER SOME TIME HAD PASSED; I NOTICED THAT ATX X WAS NOW HEADED SBOUND INSTEAD OF NBOUND AND ATX Y WAS HDG IN THE WRONG DIRECTION. I ASKED THE ACFT TO IDENT. AFTER THAT; I SAW THE CODES CHANGE BACK TO WHAT THEY WERE ORIGINALLY ASSIGNED. A RADIO XMISSION WAS MADE THAT SAID SOMETHING LIKE; 'WE WONDERED WHEN YOU WERE GOING TO CATCH THAT.' THIS IRRESPONSIBLE AND UNPROFESSIONAL BEHAVIOR SHOULD NOT BE ALLOWED IN ATC SYS. SUPPLEMENTAL INFO FROM ACN 235691: I WORKED SHIFT IN THE D AREA AT SEATTLE ARTCC. I WAS SIGNED ON AS THE MANUAL CTLR ON SECTOR 30. AT APPROX XX00 ATX X CHKED ON FREQ VFR ON TOP. THE ACFT WAS RTED FROM OAK TO PDX. AT APPROX THE SAME TIME ATX Y CHKED ON FREQ VFR ON TOP. THE ACFT WAS RTED FROM PDX TO OAK. AT APPROX XX15 TFC WAS CALLED TO BOTH ACFT. AT APPROX XX17 THE ACFT PASSED NBOUND AT 12500 FT AND ATX Y SBOUND AT 11500 FT. THE BEACONS WERE CLOSE ENOUGH TO HAVE NO RESOLUTION. WHEN RESOLUTION WAS REESTABLISHED THE DATA BLOCK FOR ATX X WAS PROCEEDING SBOUND AND THE DATA BLOCK FOR ATX Y WAS PROCEEDING NBOUND. AT APPROX XX30 THE ACFT WERE ASKED TO IDENT. ONE OF THE ACFT XMITTED 'WE WONDERED HOW LONG IT WOULD TAKE FOR YOU TO NOTICE THAT.' THE ACFT THEN WENT BACK TO THEIR ASSIGNED BEACON CODES.