NMAC BTWN A FOKKER F28 ON A VISUAL APCH TO A NON TWR ARPT JUST OUTSIDE OF THE MARKER AND A PIPER SENECA MAKING PRACTICE APCHS APPROX 300 FT BELOW ON THE SAME HEADING. THE SENECA PLTS OBSERVED THE F28 AS IT PASSED THEM OVERHEAD AND THE F28 OBSERVED THE SENECA TO THE REAR OF THEM. THE SENECA DID NOT HAVE THE XPONDER TURNED ON SO DID NOT SHOW UP ON THE F28'S TCASII.

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WE WERE ON A FLT (IFR) IN A FOKKER F28 AT FL330 FROM SEA TO BZN. WE HAD BEEN IN THE CLR MOST OF THE WAY BUT HAD TO START AN ILS RWY 12 TO GET DOWN THROUGH THE CLOUDS AT BZN. ZLC CLRED US FOR THE ILS. CTR HAD NO OTHER KNOWN TFC. WE BROKE OUT AT 9000 FT MSL. WE RPTED RWY IN SIGHT AND ZLC CLRED US FOR A VISUAL AND TO GO TO ADVISORY FREQ. WE SAW NO TFC ON OUR TCASII BUT WERE ADVISED BY BZN UNICOM THAT THERE WAS A PIPER SENECA DOING PRACTICE ILS APCHS. THE SENECA THEN RPTED HIS POS AS JUST OUTSIDE OF MANNI WHICH IS WHERE WE WERE. HE THEN SAID THAT WE HAD JUST GONE BY HIM AND THAT WE SHOULD WATCH OUT WHERE WE WERE GOING. I DID NOT SEE HIM AND DO NOT KNOW HOW CLOSE WE GOT TO HIM. I SAID OVER THE RADIO 'DO YOU HAVE A XPONDER ON?' HE DID NOT ANSWER BUT AN ACFT APPEARED ON OUR TCASII ABOUT 30 SECONDS OR SO LATER TO THE R AND BEHIND US. WE RPTED THAT WE WOULD MAKE AN UPWIND FOR A L DOWNWIND TO FOLLOW THE TFC. WE CAME IN BEHIND HIM AND LANDED. HE CONTINUED TO SHOOT APCHS. HE HAD BEEN FLYING A FEW THOUSAND FT BELOW AN OVCST ON AN ILS USED BY ACRS AND OTHER ACFT AND DID NOT HAVE HIS XPONDER ON OR NOTIFY ATC THAT HE WAS THERE. ATC WOULD NOT HAVE SEEN HIM ON RADAR BUT WE WOULD HAVE; AND DID; SEE HIS XPONDER ON OUR TCASII.