NMAC BTWN 2 ACFT THAT ATC ADVISED OF TFC TO EACH AND INSTRUCTED ONE TO MAINTAIN VISUAL SEPARATION WITH THE OTHER WHILE CLBING THROUGH THE OTHER'S ALT.

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WHILE IN CRUISE AT 12000 FT WE WERE ISSUED A TA AT OUR 10 O'CLOCK POS AND 10000 FT XING FROM L TO R. WE WERE ALSO POINTED OUT TO THE OTHER ACFT (A BE-90) AND THE PLT OF THE KING AIR RPTED US IN SIGHT AND WE ALSO ACQUIRED THE KING AIR VISUALLY AND ON TCASII. THE CTLR THEN ISSUED A CLB TO FL180 TO THE KING AIR AND TO 'MAINTAIN VISUAL' WITH THE ATR. I TOLD THE FO THAT EVEN THOUGH WE WERE VISUAL; IT WOULD BE VERY TIGHT. AS THE KING AIR CLBED; IT BECAME OBVIOUS THAT ADEQUATE SEPARATION COULD NOT BE MAINTAINED. THE KING AIR DID NOT ALTER COURSE OR CLB RATE AND I FINALLY TOOK EVASIVE ACTION BY TURNING L BEHIND THE KING AIR. WE PASSED WITHIN 500 FT OF THE KING AIR WHO STILL MADE NO MOVE TO AVOID US. HAD I NOT TAKEN EVASIVE ACTION; THERE IS NO DOUBT IN MY MIND WE WOULD HAVE HIT. THE KING AIR PLT WAS TOLD TO MAINTAIN VISUAL CONTACT WITH US IN THE CLB; YET MADE NO MOVE TO AVOID AN OBVIOUS COLLISION. IT WAS OBVIOUS THAT THE PLT NEVER LOOKED OUT THE WINDOW BECAUSE IF HE HAD; OUR ACFT WOULD HAVE FILLED HIS FIELD OF VIEW AND HE WOULD HAVE ALTERED COURSE TO AVOID US. ONE WONDERS HOW SUCH A BASIC SEE AND AVOID COULD DEGRADE INTO A NEAR MISS ON A CLR DAY; BOTH ACFT ON AN IFR CLRNC AND BOTH HAD VISUAL CONTACT WITH THE OTHER!