ALTDEV ALT OVERSHOT -- GOV FLC IS DISTRACTED DURING AN APCH BRIEFING AND DSNDS BELOW THEIR ASSIGNED ALT.

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THE OCCURRENCE/EVENT WAS QUITE SIMPLY AN 'ALT EXCURSION' WHILE ON A DSCNT CLRNC ISSUED BY ATC. AS A UNITED STATES CUSTOMS CITATION II; MODIFIED/OPERATED IN RESTR CATEGORY; WE BEGAN OUR FLT WITH A SHORT IFR PROFICIENCY FLT CONDUCTED IN VFR CONDITIONS. WE FILED AND RECEIVED OUR IFR CLRNC FROM OWH TO VCT. FROM A CRUISE ALT OF 14000 FT MSL; WE WERE GIVEN A DSCNT AT PLT'S DISCRETION TO DSND AND MAINTAIN 8000 FT. AS PIC; I CHOSE TO ALLOW THE SIC TO FLY FROM THE L SEAT ON THIS DATE; WHICH IS ALLOWED IN OUR OPS. HOWEVER; SIC DID NOT FEEL THE NEED TO PERFORM AN INST APCH TO VCT; SO I ADVISED THAT I DID. I ADVISED THE SIC THAT AT 5000 FT MSL OR APPROX 20 NM OUT; I WOULD TAKE THE CTLS AND EXECUTE THE VOR/DME RWY 12L AT VCT. THE PROB OF DSNDING THROUGH 8000 FT MSL TO APPROX 7600 FT WAS DISCOVERED BY BOTH PLTS NEARLY SIMULTANEOUSLY. AT THIS TIME ATC CALLED NOTING OUR ALT; AND VERIFYING ASSIGNED ALT AS 8000 FT MSL. WHY DID WE MISS OUR ALT? THE MODE SELECT PANEL FOR FLT DIRECTOR/AUTOPLT IS REPOSITIONED IN FRONT OF L YOKE; INSTEAD OF THE NORMAL CTR PEDESTAL AREA IN NORMAL CII'S. MAKES IT DIFFICULT TO DOUBLE-CHK. THE SIC FAILED TO ACTIVATE ALT SELECT ON FLT DIRECTOR/AUTOPLT PANEL AFTER WE SELECTED 8000 FT MSL IN VNAV COMPUTER. CONSEQUENTLY; WE DID NOT GET AN ALT AUDIO ALERT 1000 FT ABOVE PRESELECTED ALT. MOREOVER; AUTOPLT OBVIOUSLY WILL NOT CAPTURE ALT IF ALT SELECT IS NOT ARMED. AS PIC; I WAS PREOCCUPIED BRIEFING THE APCH INTO VCT; SETTING UP RADIOS; ETC. THIS KEPT ME OUT OF THE LOOP SO I DID NOT ISSUE ALT CALLS AT 1000 FT ABOVE ASSIGNED ALT; AND AT 500 FT ABOVE; PER OUR COMPANY POLICY. ALTHOUGH THIS EVENT DID NOT CAUSE A TFC CONFLICT; IT HAD THE POTENTIAL TO DO SO; ALL DUE TO A BREAKDOWN IN COCKPIT TEAMWORK/CREW COORD; SIC'S UNFAMILIARITY WITH SWITCHOLOGY FROM THE L SEAT. I'VE NEVER HAD THIS HAPPEN TO ME BEFORE; AND ONCE AGAIN; I REALIZE WE ARE ALL HUMAN AND THEREFORE ARE PRONE TO ERROR. NEXT TIME; WE WILL AVIATE FIRST; THEN NAV AND DO ALL THE OTHER LOWER PRIORITY TASKS. ALSO; I'LL TAKE SERIOUSLY A SIC'S LACK OF FAMILIARITY WITH L SEAT FUNCTIONS/SWITCHOLOGY.