PAN AM FLT 217 (AIRBUS A300, N209PA) WAS CLRD BY NEW YORK CENTER OCEANIC CTL AREA (OCA) TO PROCEED SOUTH ON AIRWAY AMBER 300 AT FL 290 OVER THE ATLANTIC OCEAN. AS IT WAS EN ROUTE, AMERICAN FLT 902 DEPARTED SAN JUAN (SJU), PR FOR A NORTHBOUND FLT ON THE SAME AIRWAY UNDER IFR CTL BY SJU COMBINED CENTER & RADAR APCH CTL (CERAP). ON-THE-JOB TRAINING WAS BEING GIVEN TO THE CERAP CTLR. AS THE CTLR'S WORKLOAD WAS GETTING 'BUSY' WITH 'A HVY RUSH HOUR,' HIS SUPVR SCANNED THE BAY TO DETERMINE THE TRAFFIC (BUT DID NOT SEE THE FLT PROGRESS STRIP FOR FLT 217) & TOLD THE TRAINEE TO ASSIGN FLT 902 TO FL 290. THE TRAINEE COORDINATED THIS WITH OCA SINCE IT WAS THE WRONG DRCTN FOR THAT ALT. OCA APPROVED & FLT 902 WAS ASSIGNED FL 290. SUBSEQUENTLY, THE 2 ACFT CONVERGED & PASSED EACH OTHER WITH AN ESTD 300' LATERAL SEPN. THE NEAR COLLISION OCCURRED IN A NON-RADAR INVIRONMENT OVER THE OCEAN ABT 200 MI NORTH OF SAN JUAN. THE OCA CTLR SAID SHE WAS AWARE BOTH FLTS WOULD BE AT THE SAME ALT, BUT APPROVAL WAS BASED ON PREMISE THAT SJU WAS AWARE & WOULD PROVIDE OTR SEPN.