FLC OF AN MDT TURBOPROP OVERSHOT DSCNT HOLDING ALT DUE TO MISCOORD OF THE ACFT CTL BTWN THEMSELVES AND NOT SETTING THE AUTOPLT PRESELECT ALT HOLD FEATURE. THE CAPT IMMEDIATELY RETURNED TO ASSIGNED ALT AFTER A 300 FT LOSS.

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WE WERE ON A VERY SHORT LEG (NORMALLY APPROX 15 MINS FLT TIME) ALL IMC AND I WAS TRYING TO OBTAIN UP TO DATE WX INFO FOR A FIELD VERY NEAR MINIMUMS AND DISCUSS WITH DISPATCH PLANS IN THE EVENT OF A MISSED APCH. THIS TOOK DISCUSSION WITH COMPANY; ATIS; AND THE LCL TWR. I WAS TRYING TO COMPLETE THIS PRIOR TO BEGINNING THE APCH AND SO WAS OFF FREQ WHEN THE FO COPIED AND BEGAN HOLDING. HE WAS ISSUED A DSCNT FROM 6000-5000 FT; BEGAN THE DSCNT BUT DIDN'T ENTER 5000 FT INTO THE AUTOPLT ALT WINDOW. AS I CAME BACK ON FREQ HE GAVE ME THE ACFT AND BEGAN THE BRIEF BUT DIDN'T MENTION THE DSCNT. IT WAS A GRADUAL DSCNT AND WHEN I NOTED THAT WE WERE DSNDING; WE WERE 300 FT BELOW 5000 FT. I SWITCHED OFF THE AUTOPLT AND IMMEDIATELY CLBED BACK TO 5000 FT. THIS ERROR WAS INDUCED BY HIGH WORKLOAD ON A VERY SHORT FLT AND OUR FAILURE AS A CREW TO FOLLOW NORMAL CRM REGARDING ALT IN GENERAL AND SPECIFICALLY WHEN CHANGING CTL OF WHO IS FLYING. I COULD HAVE AVOIDED THE PROB BY SETTING ASIDE SOME OF MY RADIO WORK; MORE CLOSELY SUPERVISING THE ENTRY TO THE HOLD; AND THEN RETURNING TO THE RADIO WHEN WE WERE ESTABLISHED.