FLC OF B737-300 ACFT; PNF FO OFF RADIO TALKING TO THEIR COMPANY WHILE PF CAPT RECEIVES AND READS BACK DSCNT CLRNC. CTLR INTERVENES WHEN SEEING ACFT DSNDING THROUGH 12000 FT. RPTR CAPT TELLS CTLR HE READ BACK 10000 FT. NO CONFLICT.

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WHILE ENRTE TO SAN IN THE VICINITY OF THE BARD VORTAC; WE ENCOUNTERED INFLT MODERATE RIME ICING AND FELT OBLIGATED TO RPT SAME TO OUR COMPANY DISPATCH. I THEN TOLD THE CAPT (PF) THAT I WOULD BE OFF THE ATC RADIO CONTACTING DISPATCH. I WAS OFF ATC RADIO FOR SEVERAL MINS ATTEMPTING CONTACT WITH OUR DISPATCH. DURING THAT TIME I OBSERVED THE CAPT PUTTING 10000 FT IN THE ALT SELECT WINDOW OF THE MCP (MODE CTL PANEL) WHICH COINCIDED WITH OUR FILED STAR (BARET4) XING RESTR FOR THE PILLO INTXN ON THE ARR. THE CAPT BEGAN DSCNT WHILE I WAS OFF THE AIR. AS SOON AS I RETURNED HE POINTED TO THE MCP AND STATED '10000 FT' AND I THEN WENT OFF ATC RADIO TO OBTAIN THE SAN ATIS AND AGAIN TO OBTAIN OUR GATE ASSIGNMENT FROM SAN OPS. BY THE TIME I RETURNED TO THE ATC RADIO WE WERE DSNDING THROUGH 11400 FT RAPIDLY AND I HEARD THE CTLR STATING SOMETHING LIKE 'ABC; SAY ALT' AND THE CAPT REPLYING OUR ALT AND DSNDING TO 10000 FT. THE CTLR SAID '12000 FT' WAS THE ASSIGNMENT BUT TO MAINTAIN '11000 FT NOW; TFC AT ...' THIS WAS ALL RATHER SUDDEN AND CONFUSING TO ME AS I HAD NOT HEARD THE ORIGINAL CLRNC BUT COULD TELL THE CTLR WAS WORRIED FROM THE TONE OF HIS VOICE. THE CAPT MADE NO ATTEMPT TO CHANGE OUR RATE OF DSCNT AND WE OBSERVED AN ACFT SOME DISTANCE TO OUR R PASSING EBOUND JUST BELOW US BUT AT WHAT SEEMED LIKE SEVERAL MI N OF OUR COURSE. I ASKED THE CAPT WHAT HAD HAPPENED AND HE OFFERED NO EXPLANATION OTHER THAN THAT HE HAD THE TFC IN SIGHT ALL ALONG. WHEN I ASKED WHAT OUR CLRNC WAS HE SAID '10000 FT' AND I ASKED IF THERE WAS ANY PROB IN THE DSCNT; HE DID NOT REPLY. NOTHING WAS SAID BY THE CTLR OTHER THAN TO HAND US OFF TO THE APCH CTLR (SOCAL APCH) AND OUR APCH AND SUBSEQUENT LNDG WERE NORMAL. I CANNOT OFFER A SOLUTION TO A PROB SUCH AS THIS; SINCE I BELIEVE THAT THIS IS A RECURRING PROB IN A 2-PERSON COCKPIT -- SIMPLY TASK OVERLOAD. WHEN ONE PERSON IS PERFORMING NON-FLYING DUTIES; THE ACFT BECOMES A SINGLE CREWED ONE.