A B757 CREW OVERSHOT ASSIGNED ALT WHILE DSNDING INTO THE NEW YORK AREA.

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CTR CLRED US TO DSND TO FL180; WHICH WE DID. CAPT WAS PF AND HIS TECHNIQUE WAS TO HAND FLY THE ACFT FROM TKOF TO LEVELOFF; AND FROM AN INTERMEDIATE ALT ON DSCNT TO LNDG; THUS HE WAS HAND FLYING THE ACFT AT THE TIME. CTR THEN CLRED US DIRECT TO A DOWN TRACK FIX. THE CAPT WENT TO FMS TO PROGRAM; MADE A MISTAKE; TRIED TO CORRECT IT; AND IN THE MEANTIME; I HAD BEGAN THE REPROGRAMMING ON MY SIDE; JUST AS WE ARE TRAINED TO DO. I STOPPED WHEN I REALIZED HE WAS DETERMINED TO DO IT ALL. MEANWHILE; THE 'HAND FLOWN' ACFT STARTED A GRADUAL DSCNT. HE GOT AS LOW AS 17500 FT BEFORE HE COMPLETED THE RECOVERY. I THEN COMPLETED THE FMS UPDATE WHILE HE CONTINUED TO HAND FLY. I HAD TRIED TO SUBTLELY INDICATE TO HIM FOR THE 3 PREVIOUS DAYS OF THE TRIP THAT IF HE CHOSE TO TURN OFF THE AUTOMATION AND HAND FLY THE ACFT; HE SHOULD PROBABLY USE THE PNF TO DO THE 'HEADS DOWN' STUFF LIKE FMS OP; BUT HE INSISTED ON BEING 'THE' PLT. THERE WAS NO PERSONALITY CONFLICT -- WE GOT ALONG GREAT; BUT PERHAPS I SHOULD HAVE BEEN MORE INSISTENT; THOUGH THIS OFTEN LEADS TO POTENTIAL CRM PROBS. SUMMARY: CAPT'S INSISTENCE ON DOING EVERYTHING PUT EVERYONE AT RISK AND ACTUALLY INCREASED THE WORKLOAD OF THE PNF. INSTEAD OF WATCHING FOR TFC OR BACKING UP THE AVIONICS; THE PNF SPENDS ALL HIS TIME BACKING UP THE CAPTS. IN THIS INSTANCE; WE MET THE INESCAPABLE RESULT OF THIS ILL-ADVISED 'TECHNIQUE.' THANK GOODNESS THERE WAS NO LOSS OF SEPARATION OR EVEN WORSE.