ZNY CTLR EXPERIENCED OPERROR AT 16000 FT WHILE DSNDING 2 ACFT TO LOWER ALTS.

Narrative

I WAS IN THE LAST HRS OF MY SHIFT WORKING A COMPLEX AND BUSY SECTOR (SECTOR 27). MIL USE OF R5802 (A/B/C) CONSTRICTED MY AVAILABLE ALTS. ZOB DID NOT PROVIDE IN TRAIL SPACE BTWN 2 PHL ARRS. ACFT X (A CITATION) DSNDING TO FL250; AND ACFT Y (A B737) LEVEL AT FL190. AN ARR TO HGR (PROP ACFT) AT FL180 ABOVE THE R5802 ACTIVE SPACE WAS TFC FOR BOTH PHL ARRS. I DSNDED ACFT Y TO 16000 FT; AND WHEN I SAW THE ACFT LEAVE 17000 FT; I DSNDED ACFT X TO 17000 FT SO THAT BOTH WOULD BE BELOW THE HGR ARR PRIOR TO HAR VOR. THEN I ATTENDED TO OTHER DUTIES IN THE SCENARIO AND BEGAN SPD CTL AND VECTORING TO ESTABLISH IN TRAIL BTWN ACFT X AND ACFT Y PHL ARRS. ABOUT 10 MI W OF HAR VOR; I GOT A RADAR HIT INDICATING ACFT Y AT ALT 17400 FT DSNDING. I IMMEDIATELY RECLRED ACFT X TO MAINTAIN FL180; THEN WENT BACK TO GET AN ALT RPT FROM ACFT Y WHO RPTED LEVEL AT 16000 FT. THIS WAS PROBABLY A 'MODE C SWAP' -- A COMMON AND CONFUSING PROB. ONCE I HAD THE RPT; I INTENDED TO RECLR ACFT X TO 17000 FT; BUT SAID 16000 FT BY MISTAKE. I CAUGHT THE ERROR AS ACFT X LEFT 16800 FT DSNDING AND HAD HIM MAINTAIN 17000 FT. NO TCASII EVENT; AS FAR AS I KNOW.