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

Narrative

I WAS WORKING LCL CTL TO 1 OF 2 PARALLEL RWYS. I HAD 5 SINGLE ENG ACFT IN THE PATTERN ALL DOING TOUCH AND GO'S. THE FIRST TIME AROUND IN THE PATTERN I POINTED EVERYONE OUT TO EVERYONE AND ISSUED THE PATTERN SEQUENCE. THE SECOND TIME AROUND; I ONLY ISSUED A SEQUENCE WITH LNDG CLRNC. THE THIRD TRIP IN THE PATTERN I AGAIN ISSUED THE SEQUENCE WITH LNDG CLRNC. AFTER CLRING A DEP FOR ON COURSE; I GLANCED OUT ON FINAL AND OBSERVED 2 SMAS STACKED ON TOP OF EACH OTHER WITH ABOUT 50 FT VERT SEPARATION. I KNOW ONE HAD BEEN CLRED FOR THE TOUCH AND GO AND I WASN'T SURE OF THE CALL SIGN OF THE OTHER (I HAD 3 SMAS IN THE PATTERN). I ISSUED A GAR TO SMA X THEN MADE A BLANKET BROADCAST THAT THERE WERE 2 SMAS ON FINAL AND THAT I DIDN'T KNOW THE CALL SIGNS BUT WERE STACKED ON TOP OF EACH OTHER. BOTH ACFT STARTED TO GAR. SMA X STARTED A SLOW CLB. SMA Y SAW HIS TFC BELOW HIM AND TOOK EVASIVE ACTION. FROM MY VIEWPOINT THERE WAS 10 TO 20 FT BTWN ACFT. IN HINDSIGHT; I SHOULD HAVE ISSUED AN IMMEDIATE TURN TO THE 1 ACFT CALL SIGN THAT I KNEW AND AN OPPOSITE TURN TO THE ACFT CALL SIGN WHICH I SUSPECTED. EVERYONE AT MY FACILITY ISSUES THE PATTERN SEQUENCE DILIGENTLY; BUT; IF THERE ARE NO CHANGES TO THE SEQUENCE; WE EXPECT THE PLTS TO REMEMBER WHO THEY ARE FOLLOWING! I PLAN TO INCLUDE THIS IN OUR NEXT RAINCHK PROGRAM AND TO TALK ABOUT IT IN OUR CTLR FBO LIAISON PROGRAM.