ZBW CTLR DURING HOLDING PROCS ASSIGNED 2 ACFT THE SAME ALT; FL190.

Narrative

I WAS HOLDING ACFT OVER THE PVD VOR. FOR BOS I WAS ATTEMPTING TO GET ACFT LOWER IN THE PATTERN. I WAS RECEIVING HDOFS THAT WERE GOING TO HOLD AS WELL AND WERE AT ALTS BELOW THE HIGHEST ACFT THAT WAS ALREADY IN HOLDING. I WAS CONTENDING WITH OVERLAPPING DATA TAGS. IT WAS DIFFICULT TO SEE THE DATA TAGS; ALTS; AND CID'S. APPROX 4-5 MINS BEFORE THE INCIDENT; I ASSIGNED FL190 TO ACR X (MD88) AND ACR Y (E145). WHEN ACR Y STARTED HIS OUTBOUND TURN; I INITIATED A VECTOR AND DSCNT TO GET SOME ALTS FOR INBOUNDS. APPROX 1 MIN AFTER I INITIATED THE TURN; ACR X ALERTED ME TO AN ACFT AT HIS ALT IN THE HOLD. ACR X WAS ON HIS INBOUND LEG. I THEN TURNED ACR Y 20 MORE DEGS L BUT THE ERROR HAD ALREADY OCCURRED. PRIOR TO THE INCIDENT; WE DEALT WITH A WX DIVERT FROM ANOTHER SECTOR; DIVERT FUEL WITHIN THE HOLDING PATTERN; RECEIVING AN ACFT THAT WAS NOT SUPPOSED TO ENTER OUR SECTOR. OVERLAPPING DATA TAGS; AND INBOUNDS ALREADY IN CONFLICT WITH THE HOLD PATTERN. PRIOR TO THIS SESSION; I WORKED APPROX 1 HR 56 MINS IN A BUSY SECTOR. MENTAL FATIGUE WAS A FACTOR. AT THE TIME OF THE INCIDENT; MY SUPVR WAS TRYING TO GET OVERTIME IN; STAFFING WAS GETTING LOW.