NMAC BTWN ACR COMMUTER AND WINGMAN OF FLT OF 2 MIL FGTS.

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2 MIL ACFT IN STANDARD FORMATION AT 12500 MSL SQ 1200 LCL TO MHR HAD A TWIN COMMUTER FLY BTWN FLT AND NEARLY MISS SECOND MIL ACFT. ESTIMATE ACR AT APPROX 12000 MSL EBOUND WHILE WE PROCEEDED WBOUND. SECOND MIL ACFT DIDN'T SEE ACR. LEAD MIL ACFT UNABLE TO WARN WINGMAN. NO EVASIVE ACTION BY ACR. WAS PROBABLY A LCL COMMUTER. WE NORMALLY HAVE RADAR FOLLOWING; HOWEVER; LCL RTBS ARE AT TIMES CONDUCTED UNDER VFR CONDITIONS. ACR WAS AT AN INCORRECT DIRECTIONAL ALT EBOUND. WILL ADVISE MIL TO EMPHASIZE RADAR FOLLOWING OR CTLED FLT IN THIS PARTICULAR VICINITY. IF ACR WAS UNDER CTLR; CTLR SHOULD HAVE SEEN THE FLTS POS AND ALT TO ADVISE ACR OF US. FORTUNATELY MY WINGMAN AND THE FOLKS IN THE ACR ARE ALIVE. WE (OUR FLT) HAVE DISCUSSED THE SITUATION AND HOW IT COULD HAVE BEEN AVOIDED. SUPPLEMENTAL INFO FROM ACN 192837. AS WINGMAN I LEFT FREQ AND WENT OVER TO MHR ATIS TO CHK ON WX. HAD TO LISTEN TWICE. WHEN RETURNED TO SQUADRON COMMON I READ WX TO LEAD ACFT. HIS RESPONSE WAS 'DID YOU SEE THAT! WE ARE GLAD YOU ARE ALIVE.' I SAID 'WHAT HAPPENED' AND WAS TOLD I HAD A NEAR MISS OF ABOUT 200 FT WITH AN ACR. I DIDN'T SEE IT BECAUSE I HAD MY HEAD DOWN WHILE WRITING ATIS AND WHEN I LOOKED UP I WAS MORE CONCERNED WITH FLYING FORMATION (LOOSE CRUISE) THAN LOOKING FOR TFC. POSSIBLY ALSO BECAUSE IT WAS HEAD-ON VERY; VERY CLR AND HAS SMALL FRONTAL AREA. THIS WAS CLOSEST CALL I'VE HAD AND I NEVER SAW OTHER ACFT. I'VE ALWAYS SAID 'IT'S THE ONE YOU DON'T SEE THAT'S GOING TO GET YOU;' AND IT ALMOST DID.