C525 PLT AND THE CAPT HAD AN ALTDEV IN PHX CLASS B.

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APCHING PHX FOR ARR; WE WERE ON VECTORS TOWARDS THE ARPT FROM THE NE. WE WERE EXPECTING A VISUAL APCH TO RWY 8L. WHILE LEVEL AT 10000 FT WE WERE GIVEN A CHANGE IN HDG AND AN ALT ASSIGNMENT OF 8000 FT. I ACKNOWLEDGED THE CHANGE IN HDG AND ALT ON THE RADIO TO ATC. THE PIC SELECTED THE NEW HDG AND NEW ALT IN THE FLT DIRECTOR. I VERIFIED THE SELECTIONS. THE AUTOPLT; WHICH WAS ENGAGED; TURNED TO THE NEW HDG BUT WAS NOT COMMANDED TO BEGIN THE DSCNT BY THE PIC. VERT SPD WAS SELECTED BY THE PIC BUT WAS NOT COMMANDED TO DSND. INSTEAD; WHEN VERT SPD WAS SELECTED THE ACFT MUST HAVE BEEN IN A SLIGHT CLB OF 100-200 FPM. THE AUTOPLT THEN CLBED THE ACFT UNTIL WE NOTICED SEVERAL MINS LATER WE WERE NOT AT OUR ASSIGNED ALT. THE REASON THIS SIT AROSE IS THAT BOTH PLTS ON BOARD WERE FOCUSED ON PROGRAMMING THE FMS FOR THE APCH WE WERE TO EXPECT. I BELIEVE THE REASON THIS BECAME A PROB IS THAT CURRENT PROC IS TO HAVE THE PF NOT ONLY FLY THE AIRPLANE BUT ALSO SELECT ALTS AND HDGS FOR THE FLT DIRECTOR AND ALSO PROGRAM THE FMS. THE PNF IS LEFT ONLY TO TALK ON THE RADIOS. I BELIEVE THE SIT COULD HAVE BEEN AVOIDED BY A BETTER DIVISION OF DUTIES. THE PF SHOULD FLY THE AIRPLANE. THE PNF SHOULD PROGRAM THE FMS WITH A VERIFICATION OF THE PROGRAMMING DONE BY THE PF.