D01 RELIEVING CTLR OBSERVES CONFLICT BTWN ENRTE LEVEL ACFT AND CLBING B767; AND ISSUES IMMEDIATE DSCNT CLRNC.

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DEPARTED DEN OFF RWY 8 ON A 065 DEG HDG; CLBING TO 10000 FT ON THE YELLOWSTONE 1; LAR TRANSITION. APCHING 10000 FT; ATC DEP GAVE US A CLRNC TO CLB TO 13000 FT AND TURN L HDG 350 DEGS. I READ BACK THE CLRNC AS I SET THE ALT IN THE WINDOW. BOTH THE FO AND I POINTED AT IT. APPROX 12700 FT; A CTLR SAID 'XXX TURN R 030 DEGS AND DSND TO 12000 FT; TFC 12 O'CLOCK; 4 NM AT ONE THREE THOUSAND.' WE COMPLIED IMMEDIATELY WITH THE CLRNC AND SIMULTANEOUSLY RECEIVED A TA ON OUR TCASII. THE VOICE OF THIS CTLR WAS NOT THE SAME ONE THAT HAD GIVEN US THE INITIAL TURN AND CLB CLRNC. WHEN I ASKED WHAT HE SHOWED AS OUR ASSIGNED ALT; THE CTLR RESPONDED 'ONE TWO THOUSAND.' HE ALSO SAID HE HAD JUST TAKEN OVER FROM THE PREVIOUS CTLR. BOTH THE FO AND MYSELF UNDERSTOOD ONE THREE THOUSAND. OUR JUMP SEAT RIDER SAID HE HEARD ONE TWO THOUSAND BUT WHEN I READ BACK ONE THREE THOUSAND WITH NO CONCERN FROM THE CTLR; HE THEN THOUGHT IT HAD BEEN ONE THREE THOUSAND. DID BOTH COCKPIT AND ATC MISUNDERSTAND EACH OTHER'S 'TWO AND THREE?' OR DID THE SHIFT CHANGE PLAY A ROLE? THE FACT IS; SEPARATION WAS LOST; BUT THANKS TO AN ALERT CTLR; MORE SERIOUS PROBS WERE AVERTED. SUPPLEMENTAL INFO FROM ACN 520422: ATC DEP GAVE US CLRNC TO ONE THREE THOUSAND AND A L TURN TO 350 DEGS. THE CAPT READ BACK CLRNC AND SET 13000 FT IN THE ALT WINDOW WHICH WE BOTH ACKNOWLEDGED.