SMA X HAD POSSIBLE NMAC WITH SMA Y ON FINAL APCH TO LNDG. SEE AND AVOID CONCEPT.

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WHILE WAITING FOR DEP ON RWY 26 AT DXR I OBSERVED SMA X ON LONG FINAL. AN SMT IN FRONT OF ME DEPARTED AND I BECAME #1 HOLDING SHORT. I AGAIN LOOKED AT THE FINAL APCH PATH TO SEE SMA X ON ABOUT A 1/8 MI FINAL AT AROUND 100 FT AGL AND WATCHED AS ANOTHER SMA Y TURNED L BASE TO FINAL IMMEDIATELY OVER THE FIRST ACFT. AT THAT POINT IT WOULD HAVE BEEN IMPOSSIBLE FOR EITHER ACFT TO SEE THE OTHER. I COULD NOT DISTINGUISH THE AIRPLANES AND WAS AFRAID THAT ANY XMISSION TO THEM WOULD HAVE CAUSED THE LOWER ACFT TO CLB INTO THE UPPER PLANE. I TRANSMITTED; 'TWR YOU HAVE 2 ACFT ON SHORT FINAL.' AFTER A PAUSE; THE TWR FOLLOWED WITH INSTRUCTIONS FOR 'SMA Y; GAR.' THE UPPER ACFT THEN CLBED AND MOVED TO THE R SIDE OF THE RWY FOR THE GAR. I DO NOT KNOW HOW THE TWR KNEW WHICH ACFT WAS ON TOP; OR IF IT WAS A LUCKY GUESS. MUCH HAS BEEN WRITTEN ABOUT CAUTION IN THE TFC PATTERN AT UNCTLED ARPTS -- IT MUST BE EMPHASIZED TO PLTS THAT SUCH VIGILANCE NEEDS TO BE UTILIZED AT CTLED ARPTS ALSO. CERTAINLY THE AIRPLANE GOING FROM BASE TO FINAL DID NOT SEE THE ACFT ON FINAL. I DID NOT NOTICE THE PREVIOUS TWR COMS SO I CANNOT COMMENT ON IT. HOWEVER; IT WOULD APPEAR THAT THEIR SEQUENCING OF LNDG ACFT HAD A PROBLEM SOMEWHERE.