A DSNDING EMB120 OVERSHOT ITS ALT WHEN THE CREW FAILED TO COORD THE FLT'S ALT ASSIGNMENT WITH EACH OTHER.

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THE CAPT WAS FLYING THE ACFT FROM FAT TO SFO. WE JUST PASSED OVER THE MOD VOR MAKING OUR L TURN TO INTERCEPT THE 245 DEG RADIAL WHEN STOCKTON APCH ASKED US TO DSND FROM 11000 FT TO 9000 FT. I READ BACK THE CHANGE IN ALTS. WE WERE BOTH TALKING ABOUT AN UNRELATED SUBJECT AND I WAS WORKING ON COMPANY PAPERWORK. THE CAPT DIDN'T CHANGE THE ALERTER TO 9000 FT. AFTER A FEW MINS STOCKTON CAME BACK AND AMENDED OUR ALT TO 10000 FT. (THE CAPT HAD NOT STARTED THE DSCNT TO 9000 FT YET.) I READ BACK THE NEW ASSIGNED ALT BUT THE CAPT DID NOT HEAR IT. I THEN PROMPTED THE CAPT FOR THE DSCNT AT THE SAME TIME HE SET THE ALT ALERTER FOR 9000 FT. HE DIDN'T HEAR THE NEW ASSIGNED ALT OF 10000 FT. I; THE PNF; DIDN'T CONCUR THE ALT ALERTER ASSIGNED AMENDED ALT OF 10000 FT. THE CAPT THOUGHT THE ALT WAS STILL 9000 FT. I NEEDED TO GET THE ATIS FOR SFO; MY HEAD WAS DOWN WRITING THE INFO; WHEN I NOTICED US GOING THROUGH 9500 FT; WHEN I BROUGHT IT TO THE ATTN OF THE CAPT. WE STOPPED THE DSCNT AT APPROX 9400 FT; +/-100 FT. AT THIS TIME; STOCKTON APCH ASKED US TO CONTINUE THE DSCNT TO 9000 FT. THE CTLR DID NOT MENTION ANYTHING TO US REGARDING THE ALT INCURSION. IT WAS A CREW ERROR DUE TO OUR TALKING AND OUR LACK OF NOT FOLLOWING COMPANY SOP FOR ALT ALERTER PROCS. AS A CREW; WE TALKED ABOUT THE SIT AND WE BOTH AGREED TO FOLLOW THE COMPANY'S SOP. IF WE HAD FOLLOWED COMPANY SOP THE SIT WOULD NEVER HAVE HAPPENED.