LESS THAN STANDARD SEPARATION AND NMAC BETWEEN SMALL GA AND TWIN GA ACFT. OPERATIONAL ERROR.

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OCCURRED ON IFR FLT FROM ROA TO SAV. CLRD FROM ROA; V103 GSO; DIRECT CTF AT 8000 ETC. OVER GREENSBORO I WAS HANDED OFF TO GREENSBORO DEP ON 126.6. SEVERAL MINUTES LATER; AT XA15; 2 DME S OF GSO VOR ON 193 DEG RADIAL; ATC CALLED TFC AT MY 9 O'CLOCK POSITION. IMMEDIATELY I LOOKED LEFT TO DISCOVER A LIGHT TWIN AT MY 8:30 POSITION APPROX 50' BELOW MY ALT AND APPROX 300' FROM MY ACFT CLOSING AT AN ANGLE DIRECTLY AT MY ACFT. THE TWIN PASSED IMMEDIATELY BEHIND MY ACFT AND TURNED LEFT PARALLEL TO MY ROUTE OF FLT AT MY ALT. HIS RESULTING ROUTE OF FLT AND ALT WERE THE SAME AS MINE AND NOW APPROX 1/2 MI W OF ME. UPON FIRST SIGHTING I HAD CALLED A 'NEAR MISS' TO GREENSBORO; THEN REPORTED HIS SUBSEQUENT FLT AS HE THEN TURNED LEFT IN FRONT OF ME. GSO DEP THEN ADVISED THAT THE TWIN HAD HAD ME 'IN SIGHT' AND WOULD CONTINUE AHEAD OF ME AT 8000' ON R193 DEG. I DECLARED THAT I SHOULD HAVE BEEN ADVISED OF HIS PROX EARLIER. DEP STATED THEY WERE TRYING. NO DELIBERATE EVASIVE ACTION HAD BEEN TAKEN AS THE INCIDENT HAD OCCURRED TOO QUICKLY. ALSO; ANY INCORRECT EVASIVE ACTION MIGHT HAVE RESULTED IN A COLLISION. AT APPROX XA25 GREENSBORO ATC CALLED THE LIGHT TWIN AND ASKED IF HE HAD HAD THE SINGLE. HE PASSED BEHIND 'IN SIGHT' UPON PASSING. NO AUDIBLE RESPONSE WAS HEARD ON 126.6. I DOUBT THE TWIN HAD ME IN SIGHT. CLEARLY; SEPARATION WAS COMPROMISED. I BELIEVE THE OCCURRENCE RESULTED FROM ATC'S FAILURE TO DIRECT THE TWIN TO A SAFE SEPARATION FROM ME AND THE TWIN PLT'S FAILURE TO OBSERVE 'SEE AND AVOID' AND REQUIRED SEPARATION MINIMUMS. SOLUTION: PERHAPS A REVIEW OF SEPARATION MINIMUMS WITH GSO ATC FACILITY WITH RESPECT TO EARLY ADVISORIES OF TFC CONFLICTS. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING INFORMATION: RPTR DID NOT STATE PRECISELY THAT HE WISHED TO FILE AN NMAC RPT; AND HE WAS NOT MET BY ANY FAA REP AT DEST ARPT. I EXPLAINED PROC FOR FILING OFFICIAL NMAC RPT. ACFT Y WAS OFF GSO ARPT AND RPTR PRETTY SURE Y WAS ON IFR CLRNC AT 8000'. ACFT Y QUICKLY PULLED AWAY FROM RPTR ACFT X AND SOON WAS GONE TO ANOTHER FREQ. RPTR THINKS ATC TRIED TO CONVINCE HIM THAT VISUAL SEP WAS BEING APPLIED; BUT RPTR DOES NOT THINK ACFT Y PLT SAW ACFT X UNTIL THEY ALMOST HIT.