A B757 FLC CLBS ABOVE THEIR ASSIGNED ALT WHEN THE PF; THE PIC; SETS HIS OWN ALT ALERT FOR THE OTHER RPTED TFC'S ALT 25 MI SE OF PSP; CA.

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WE DEPARTED SNA VIA THE FMS SID AND RECEIVED VECTORS TO THE TRM VOR WITH A CLRNC TO CLB TO 13000 FT. WE WERE SOON CLRED TO 14000 FT AND PRIOR TO REACHING 14000 FT; ASSIGNED 15000 FT. AS THE PNF; I SET IN THE ALT WINDOW. INCLUDED WITH THE CLRNC TO CLB TO 15000 FT WAS A TA FOR AN ACFT AT 16000 FT AND 2 O'CLOCK POS. I DID NOT CALL OUT '14000 FT FOR 15000 FT' AS I READ BACK 'ONE-FIVE THOUSAND' TO ATC AND LOOKED FOR TFC. WHILE LOOKING FOR THE TFC; THE CAPT CALLED OUT '15000 FT FOR 16000 FT.' I LOOKED BACK AT THE ALT WINDOW AND SAW 16000 FT SET. THE CAPT HAD RESET THE ALT TO 16000 FT THINKING THAT WAS THE ALT WE WERE CLRED TO; MISTAKING THE OTHER TFC ALT FOR OUR ASSIGNED ALT. THERE WAS A LAG IN MY ACTIONS AS I WAS CONFUSED AS TO 'WHO' HAD SET 16000 FT IN THE WINDOW OR IF IN FACT THIS ALT WAS CORRECT. WE WERE SOON INFORMED THAT WE SHOULD HAVE BEEN AT 15000 FT; NOT 16000 FT. NO CONFLICT OCCURRED. THE CAPT SHOULD NOT HAVE RESET THE ALT WITHOUT CHKING WITH ME OR MORE PROPERLY; LETTING THE PNF SET ALTS; AS IS OUR POLICY. HOWEVER; MY NOT CALLING OUT '14000 FT FOR 15000 FT' DUE TO THE ATC READBACK AND HIGH RATE OF CLB COULD HAVE REINFORCED TO THE CAPT THAT WE WERE INDEED CLRED TO 16000 FT. SUPPLEMENTAL INFO FROM ACN 481356: THE CLRNC THAT I MISTAKENLY THOUGHT WAS FOR US; WAS ACTUALLY FOR OUR TFC TO MAINTAIN 16000 FT. THERE WAS A NEARLY CONSTANT STREAM OF CLRNCS TO MANY ACFT IN THE AREA AND I SIMPLY DID NOT QUESTION THIS ONE CAREFULLY ENOUGH BEFORE STARTING THE CLB. THE CTLR LEVELED THE OTHER ACFT AT 17000 FT AND NO NEAR MISS OCCURRED.