OKC CTLR TRANSPOSES MIL TRAINING CALL SIGNS ISSUING INCORRECT CLRNCS; RESULTING IN A LOSS OF SEPARATION.

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MIL TRAINING FLT; A FORMATION OF 2 BE40'S; CALL SIGN 'MTR2X FLT;' NEEDED TO SPLIT UP AND GET SEPARATE IFR CLRNCS TO PENETRATE THE WX AND LAND AT OKC. FROM 10000 FT MSL AND IN VMC; EACH ACFT WAS GIVEN A SEPARATE SQUAWK AND A VECTOR TO SEQUENCE US IN TO THE ACTIVE RWY 17R. CONFUSION BY THE CTLR (I BELIEVE) ABOUT WHO WAS WHERE IN THE FORMATION; BROUGHT US TO AN NMAC. WHEN THE SEPARATE CLRNCS WERE ISSUED AND SQUAWKS SET; MTR2X WAS LEAD ACFT AND MTR1 WAS THE WINGMAN. MTR2X TOOK VECTORS TO THE ILS RWY 17R; WHILE MTR1Y TOOK VECTORS FOR A VISUAL TO RWY 17R. A LATE TURN TO FINAL AND CLRED 3000 FT MSL; MTR2X BROKE OUT OF THE WX IN A DSCNT THROUGH 4500 FT MSL. I (MTR1Y) REACTED TO A TCASII WARNING AND LEVELED OFF. WHILE I CONTINUED TO INTERCEPT FINAL (SBOUND); A TWIN PROP ACFT PASSED UNDERNEATH ME (EBOUND) BY 200 FT (PER TCASII). I ESTIMATE THE TWIN WAS AT 4000 FT MSL. NO CALL FOR TFC OR AVOIDANCE VECTORS WERE EVER GIVEN BY OKC APCH. WHEN APCH SWITCHED MY WINGMAN (4 MI OR SO IN TRAIL OF ME) TO TWR BEFORE THEY SWITCHED ME (I ASKED TO BE SWITCHED INSIDE THE FAF ON APCH); I REALIZED THERE MUST HAVE BEEN A CALL SIGN MIX-UP ABOUT WHO WAS IN FRONT. THE SUPVR FOR OKC APCH WAS NOTIFIED BY PHONE ON THE GND OF THE INCIDENT.