LESS THAN STANDARD SEPARATION BETWEEN CORP ACFT AND ANOTHER UNKNOWN TYPE IFR ACFT. OPERATIONAL ERROR.

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WHEN AT 6000' 20 NM SE OF RFD VOR AND ASSIGNED HDG 270 DEGS BY RFD ATC THE RFD ATC WITH SOME VIGOR TOLD ME TO TURN RIGHT TO 360 DEGS AND ANOTHER ACFT TO TURN LEFT TO 270 DEGS. THE OTHER WAS APPARENTLY AT 6000' IFR AND HDG ABOUT 020 DEGS. I RESPONDED IMMEDIATELY AND VIGOROUSLY BY TURNING RIGHT TO 360 DEGS. THE OTHER ACFT RESPONDED BY TURNING LEFT TO 270 DEGS. WHEN LEVELING ON 360 DEGS I SAW THE OTHER ACFT TURNING TO 270 DEGS. I SO ADVISED ATC OF MY VISUAL CONTACT WITH THE OTHER ACFT. ATC SAID; 'MAINTAIN VISUAL SEP AND TURN DIRECT TO DBQ.' I TURNED DIRECT DBQ ABOUT 300 DEGS. ATC TOLD THE OTHER ACFT TO TURN ON COURSE; I GUESS 020 DEGS. I MAINTAINED VISUAL CONTACT WITH THE OTHER ACFT. AS I APCHED THE OTHER ACFT TO PASS BEHIND THE PLT APPARENTLY SAW ME FOR THE FIRST TIME AND RESPONDED WITH A CLB AND A COMMENT THAT 'WE WOULD HAVE HIT IF I DIDN'T CLB.' I WAS IN A SLIGHT LEFT TURN AND DSNDED 100'. IT WAS IMMEDIATELY APPARENT THAT A DSCNT WAS NOT NECESSARY. EVEN WITH THE SLOWER AIRSPD OF THE SECOND ACFT CLBING I STILL PASSED BEHIND THE OTHER ACFT BY 2 MI. I THINK ATC SHOULD HAVE TURNED THE OTHER ACFT LEFT TO 020 DEGS INSTEAD OF RIGHT TO 020 DEGS.