B757 LNDG JAC EXPERIENCED GAR BECAUSE OF UNKNOWN TFC AND EXCESS ALT.

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APPROX 50 MI S OF JAC; SLC CTR INFORMED US THAT ANOTHER ACFT WAS SHOOTING A PRACTICE ILS APCH TO RWY 18 AT JAC; AND ASKED US TO SLOW DOWN TO AVOID HOLDING. WE SLOWED AND CHANGED OUR REQUEST FROM VOR-DME 36 WITH A VISUAL 18 TO THE ILS RWY 18 TO AVOID A DELAY. WE WERE CLRED DIRECT DUNIOR FOR THE ILS NOPT TO RWY 18. PRIOR TO DUNIOR; CTR GAVE HOLDING INSTRUCTIONS FOR QUIRT WITH AN EXPECTED FURTHER CLRNC OF XA15Z. SHORTLY THEREAFTER; THE OTHER ACFT; A LIGHT TWIN; CANCELED IFR; AND WENT TO ADVISORY FREQ AND LEFT CTR'S RADAR COVERAGE OF 13000 FT MSL. CTR THEN CANCELLED OUR HOLDING AND CLRED US TO INTERCEPT THE ILS TO RWY 18; AND SAID 'CLRED THE ILS APCH RWY 18.' AT THIS TIME; WE DID NOT HAVE THE PRECEDING ACFT IN SIGHT OR THE RWY IN SIGHT. WE CONTINUED ON THE APCH. AFTER BEING HANDED TO TWR; WE ASKED WHERE THE OTHER ACFT WAS; AND DISCOVERED THAT THE PLT HAD NOT SPOKEN TO TWR YET. TWR HAD NO IDEA WHERE HE WAS. AT 10 MI ON FINAL; WE RECEIVED A YELLOW TCASII ADVISORY THAT THE LIGHT TWIN WAS .1 NM BEHIND; AND 700 FT BELOW. AT THIS TIME; WE WERE DSNDING RAPIDLY TO CATCH UP. WE INITIATED A GAR AND LANDED UNEVENTFULLY. AFTER MUCH DISCUSSION; THE CAPT AND I DECIDED EVERYTHING WAS DONE BY THE BOOK; BUT MORE COULD HAVE BEEN DONE BY THE CTLR TO RELIEVE A POSSIBLE CONFLICT.