A320 CREW HAD AN ALT OVERSHOT IN ORD CLASS B.

Narrative

WE WERE CLRED DIRECT ORD DSNDING TO 7000 FT. I WAS THE PNF AND THE FO WAS THE PF. THE FO DISENGAGED THE ACFT AND SELECTED VS ON THE FCU HIMSELF. ATC GAVE US A TURN TO THE DOWNWIND AND; WHILE I WAS SEQUENCING THE FMC FOR THE APCH; THE FO SELECTED OPEN DES ON THE FCU (NON SOP). I BELIEVE HE INADVERTENTLY CHANGED THE ALT TO 6000 FT AND NEITHER ONE OF US NOTICED THE ALT CHANGE. I WAS DISTRACTED WITH THE FMC AND HE WAS DISTRACTED BY HAND FLYING WHILE CHANGING THE FCU. GOING THRU 6700 FT ATC ASKED ABOUT OUR CLRED ALT. AT THE SAME TIME WE HAD A TCASII TFC ALERT FOR AN ACFT 1600 FT BELOW. I STATED WE WERE CLRED TO 6000 FT AND ATC STATED WE WERE CLRED TO 7000 FT BUT CONTINUE TO 6000 FT. I SHOULD HAVE EMPHASIZED TO THE FO TO CALL FOR FCU CHANGES WHILE HAND FLYING PER SOP. NO LOSS OF SEPARATION OCCURRED. IT ONLY SPENT ABOUT 7-8 SECONDS TO SEQUENCE THE FMC FOR THE APCH AND IN THAT SHORT TIME THE ERROR OCCURRED. I'VE BEEN A CAPT ON THE A320 FOR ALMOST 7 YEARS AND THIS IS THE FIRST TIME AN ERROR LIKE THIS OCCURRED. I ALSO THINK A BETTER DESIGN FOR THE FCU WOULD BE TO HAVE A SEPARATE SWITCH FROM THE ALT SELECTOR TO CHANGE DSCNT MODES (LIKE FLT CREWH ON THE 767.)