N90 CTLR EXPERIENCED OPERROR AT 6000 FT DURING COMBINED POS OPS.

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WORKING COMBINED POS AT N90; LAGUARDIA SECTOR; WESTCHESTER ARRS AND DEPS AND LGA ARRS. TSTMS IN AREA. SEVERAL ACFT DEVIATING DUE TO WX. I WAS HOLDING 3 ACFT FOR LGA; AS ACFT WERE NOT LNDG DUE TO TSTM ON THE APCH. THE ACFT WERE BEING VECTORED AROUND WX. I WAS ALSO WORKING 2 HPN ARRS; AND 4 THROUGH FLTS; ONE OF WHICH REQUIRED MUCH COORD DUE TO HIS WX DEVS. THE BARON WAS ON A FLT THROUGH MY AREA FROM BDL TO TEB. THE B190 WAS BEING DELAYED VECTORED DUE TO WX AT LGA. A COUPLE OF MINS PRIOR TO THE INCIDENT; LGA STARTED TO ACCEPT ARRS; THE B190 WANTED TO WAIT A BIT. WHILE THE OTHER 2 LGA ACFT I HAD STARTED BEING TURNED BACK TOWARDS THE ARPT. THOSE 2 ACFT WERE AT 4000 FT AND 5000 FT; WHILE THE B190 WAS ABOVE THEM AT 6000 FT; ON A NW HDG. THE BE58 WAS WBOUND THROUGH MY AIRSPACE AT 6000 FT. HE USUALLY IS DSNDED TO 4000 FT PRIOR TO LEAVING MY AIRSPACE. I COULD NOT DSND HIM TO A LOWER ALT AS THE 2 HPN ARRS WERE LATE DSNDING OUT OF 6000 FT ON THE APCH TO HPN. THE DEVIATING ACFT HAD DEPARTED LGA FOR DFW; YET WAS NEBOUND; 40 NM NE OF LGA TRYING TO GET AROUND WX. I NEEDED TO COORDINATE WITH 2 OTHER SECTORS AND ZBW ABOUT HIS DEV NEEDS AND DESIRES. THE DISTRS OF THIS; ALONG WITH COORDINATING THE START OF RUNNING APCHS BACK INTO LGA LED TO MY FAILURE TO SPOT THE CONFLICT AND TAKE APPROPRIATE ACTION IN TIME TO ENSURE ADEQUATE SEPARATION. IN THE FUTURE; AN ADDITIONAL PERSON AROUND TO COORDINATE WILL BE USED!