2 MIL A10'S IN FORMATION FLT PASS IN CLOSE PROX TO A COMMUTER ACFT AT 14000 FT. TFC WAS ISSUED ON 1 MIL ACFT OBSERVED ON TCASII. THE COMMUTER PLT SAW THE SECOND A10 WITHOUT AN OPERATING XPONDER BEHIND AND BELOW THE LEAD ACFT. THEY PASSED WITHIN 1000 FT OF EACH OTHER.

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APPROX 5 MINS AFTER LEVELING OFF AT CRUISE ALT; ATC CALLED OUT VFR TFC TO THE FO WHILE I WAS OFF THE RADIOS MAKING A ROUTINE PAX BRIEFING. WHEN I RETURNED TO THE RADIO FREQ THE FO POINTED TO TFC DISPLAYED ON THE TCASII DISPLAY. THE TFC WAS JUST INSIDE THE 20 MI RANGE AT 500 FT ABOVE OUR ALT. THE FO INFORMED ME THAT ATC TOLD HER THE TFC WAS NBOUND (WE WERE WBOUND); HOWEVER; THE TFC ON THE TCASII APPEARED TO BE MOVING E. DUE TO THE DISTANCE FROM THE TFC I COULD NOT ACQUIRE THEM VISUALLY. AFTER VISUALLY SEARCHING FOR THE TFC; (IT WAS AT THE 10-11 O'CLOCK POS) I BEGAN TO ATTEND TO OTHER PNF DUTIES; IE; LOOKING AT THE CHARTS AND TUNING RADIOS. ABOUT 5 MINS AFTER THE TA FROM ATC; (THERE WERE NO SUBSEQUENT ADVISORIES) THE TCASII PROVIDED A TFC WARNING FOR AN ACFT AT 9 O'CLOCK AND +500 FT. I VISUALLY ACQUIRED THE ACFT AT 500 FT ABOVE AND SAW THAT IT WAS AN A10 AFTER WHICH I SAW ANOTHER IN LOOSE FORMATION TRAILING THE LEAD AT ABOUT 100 FT ABOVE US. THE FO ALSO SAW THE FORMATION ACFT AND BEGAN A DSCNT. AT THAT MOMENT THE WINGMAN TURNED PARALLEL TO OUR COURSE AND DROPPED BEHIND US WHILE THE FORMATION LEADER PASSED OVERHEAD. THE CONFLICT WAS CAUSED BY THE WINGMAN (WITH HIS XPONDER/ENCODER TURNED OFF) FLYING SEVERAL HUNDRED FT LOWER THAN THE LEAD ACFT. ATC HAD NO WAY OF KNOWING THIS WOULD CAUSE A CONFLICT SINCE THEY ONLY SAW AND SPOKE WITH THE LEADER WHO WAS AT THE PROPER ALT. IN THE FUTURE I WOULD RECOMMEND GREATER SEPARATION (VERTLY) FROM FORMATION FLTS SINCE ONLY 1 OF THE ACFT IS IN RADAR CONTACT. FROM NOW ON I WILL HAVE A GREATER AWARENESS OF THE OTHER ACFT IN A FORMATION FLT WHEN A FORMATION IS CALLED OUT AS A POTENTIAL TFC CONFLICT.