PLT OF GA SMA ACFT HAD NMAC WITH ANOTHER ACFT UNKNOWN DURING DSCNT CRUISE TO LAND.

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INBOUND FROM IOC TO APA; CALLED TWR AT IOC AND WAS TOLD TO FOLLOW T-TAIL (SMA #1) (WHO HAD INCORRECTLY RPTED POS) WHICH WAS IN SIGHT. AFTER LNDG; PLT OF SMA #2 FIXED GEAR SINGLE; INFORMED ME WE HAD NEARLY COLLIDED. I HAD PASSED ON HIS L 'ABOUT 50 FT.' MY TCASII DID INDICATE A TARGET AT 0.1 NM THAT I DIDN'T SEE. THAT INDICATES PLUS OR MINUS 500 FT SEPARATION; BUT HE SAID 50 FT; MY SPD ABOUT 50 KIAS FASTER. I NEVER SAW HIM AS WAS LOOKING AT 11 O'CLOCK POS AT SMA #1. I DOUBT THE 50 FT; BUT TWR SHOULD HAVE CALLED BOTH TARGETS TO ME; NOT JUST THE SMA #1. THEY WERE NOT TALKING TO SMA #2. APA TWR DOES NOT CALL TFC WELL AND HAS NOTICED A SEVERE DETERIORATION IN TFC CTL TWR IN RECENT MONTHS. I HAVE BEEN FLYING BASED AT APA FOR 11 YRS. I HAD 2 CONFLICTS THIS DAY. THE OTHER IS ON A SEPARATE FORM. MANY PLTS ARE COMPLAINING. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: LEFT WORD WITH THE RPTR THAT THE AIRSPACE WITHIN HIS AREA OF CONFLICTING TFC WAS NOT IN THE APA TWR CTL BUT IN THE DEN APCH AIRSPACE AND OUTSIDE OF THE TCA WHEREIN; WHEN VFR; THE SEE AND AVOID CONCEPT IS USED. THEREFORE; HIS COMMENT REGARDING THE APA TWR WITH REGARD TO THEIR CTL EFFECTIVENESS WAS NOT APPROPRIATE SINCE THEY WOULD HAVE NO KNOWLEDGE; EXCEPT WHEN THE ACFT RPT TO THE TWR AND GIVE THEIR POS AS DID THE #1 ACFT GIVEN AS AN ADVISORY.