NMAC DURING A SCENIC TOUR AND CLB OUT.

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FAR 135 IOE LINE CHK. OUR 402C AND THE SECOND ACFT A TWIN OTTER; CONVERGED JUST E OF BOULDER VOR AT 5000 FT AND 5500 FT RESPECTIVELY. THESE ALTS ARE THE FINAL SEGMENT ALTS FOR THE 2 SEPARATE CODED VFR DEP PROCS WE WERE FLYING. I WAS PIC; BUT WAS NOT THE PF. THE PF WAS IN THE L SEAT; I WAS IN THE R. WE PASSED 500 FT BENEATH THE OTTER AS THE PROCS PROVIDED; WE WERE HDG 150 DEGS; THE OTTER 065 DEGS AS WE CROSSED. EACH ACFT WAS THEN TO PERFORM A TURN AROUND HOOVER DAM IN ORDER TO DISPLAY THIS OUT THE L SIDES OF THE 2 ACFT. THE FINAL CRUISE ALT FOR EACH ACFT WAS 7500 FT; AND OUR GND TRACKS WERE TO CONVERGE E OF THE DAM. I ASKED THE PF WHETHER HE HAD THE OTTER IN SIGHT AS WE BOTH TURNED L AROUND THE DAM AND BEGAN OUR CLBS. HE SAID HE DID; AND IN FACT I COULD ALSO SEE THE OTTER FROM MY POS IN THE R SEAT THROUGH THE PF'S SIDE WINDOW; THEN OUR WINDSHIELD. I TOLD THE PF TO TURN R AND GIVE THE OTTER MORE ROOM. HE DID; AND WE CONTINUED OUR CLBS; MORE OR LESS ON THE SAME; BUT PARALLEL COURSES. WE LEVELED OFF AT 6500 FT MOMENTARILY AND CALLED OUR POS ABEAM; ON THE R AND ABOUT 800 FT LOWER THAN THE OTTER. THE OTTER CREW ACKNOWLEDGED US 'IN SIGHT.' WE RESUMED OUR CLB; HOWEVER I COULD NO LONGER SEE THE OTTER DUE TO MY POS ON THE R SIDE OF THE ACFT. I IMAGINED THAT THE OTTER WOULD FALL BEHIND; AS WE WERE FASTER; AND ASSUMED THE PF STILL HAD THE OTHER ACFT IN SIGHT. CLBING THROUGH 7200 FT OR 7300 FT; I ASKED THE PF IF HE STILL HAD THE OTTER IN SIGHT ON HIS SIDE. AS I DID SO; I LEANED OVER TO THE L AND LOOKED UP AND OUT OF THE PF'S SIDE; THEN LOOKED STRAIGHT-UP THROUGH OUR WINDSHIELD. THE OTTER WAS DIRECTLY ABOVE US; 200-300 FT. I TOOK THE CTLS; AND WITHOUT ANY ABRUPT MANEUVERING; ARRESTED OUR CLB AND BEGAN A SMOOTH DSCNT AND TURNED R 30 DEGS TO SEPARATE OURSELVES HORIZLY AS WELL. SOON THE OTTER BECAME VISIBLE BEHIND AND SLIGHTLY ABOVE US AT ABOUT 1/2 MI. WE CONTINUED THE FLT TO OUR DEST WITHOUT FURTHER INCIDENT.