THE FLC FAILED TO DSND IN ACCORDANCE WITH ATC CLRNC.

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IT WAS MY FIRST DAY BACK ON MY USUAL GLASS COCKPIT AIRPLANE AFTER MORE THAN 2 WKS AWAY. (I WAS CALLED UP BY MY EMPLOYER TO FLY A TRIP ON A B-747 MY COMPANY BELIEVES IN DUAL QUALIFICATION.) I WAS GETTING MY SEA LEGS BACK IN THE A-320 BY FLYING MY TRIP FROM MSP TO MIA; THEN MIA TO DTW. ON THE WAY N; WE WERE AT 37000 FT AND THE CTLR ISSUED US A CLRNC TO 33000 FT. THE COPLT; WHO WAS THE PF; DIALED IT IN ON THE FCU; BUT NEGLECTED TO PULL THE KNOB TO INITIATE THE ALT CHANGE. I MISSED THIS MISTAKE BECAUSE I WAS USED TO 'STEAM GAUGES' BECAUSE OF MY RECENT FORCED ASSIGNMENT TO THE B-747; AND WAS NOT YET SHARP AS I USUALLY AM ON 'GLASS.' THE CTLR ASKED US LATER WHY WE HADN'T STARTED DOWN; AND SOUNDED PEEVED. THE FO WAS EMBARRASSED HE HADN'T PULLED THE KNOB; AND I WAS EMBARRASSED I HADN'T CAUGHT IT. WE STARTED DOWN; WITH APOLOGIES TO ATC. HERE'S MY POINT: I DON'T KNOW WHY THE FO DIDN'T PULL THE KNOB; BUT I DO KNOW WHY I DIDN'T CATCH IT. I AM 'DUAL QUALIFIED' AND AM WORKING HARD TO DO AN AVERAGE JOB ON 2 AIRPLANES; INSTEAD OF DOING A GREAT JOB ON 1! DUAL QUALIFICATION SHOULD NOT BE ALLOWED. AT THE VERY LEAST; IF IT MUST BE ALLOWED FOR DIRE ECONOMIC REASONS AT AN AIRLINE; THEN IT SHOULD ONLY BE ALLOWED BTWN 2 GLASS AIRPLANES; OR 2 STEAM GAUGE AIRPLANES; BUT NEVER BTWN 1 STEAM AND 1 GLASS! MY 2 DUAL QUALIFICATION ACFT ARE AN AIRBUS A-320 AND A B-747; AND YOU CAN'T GET MORE DIFFERENT THAN THAT! NOT GOOD!