LESS THAN STANDARD SEPARATION BETWEEN 2 ACR ACFT. SIMILAR CALL SIGN PROBLEM. OPERATIONAL ERROR.

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OUR FLT X (IHH) EITHER NEXT INTRAIL OR SECOND TO A COMPANY W (IIH) ARRIVING MIA AREA FROM SE WITH IDENTICAL ROUTING IN THIS PHASE; IDENTICAL CLRNCS; ETC. GREAT POSSIBILITY OF CONFUSION BECAUSE OF SIMILAR FLT NUMBERS. OUR FLT X CLRED TO 6000' AND SUBSEQUENTLY TO 3000' WITH HDG CHANGE AFTER PASSING BSY VOR TO 280 DEGS. FOREIGN ACR Y WAS OUTBND FROM MIAMI AT 5000'; 180 DEG HDG. APPEARS TO BE CLASSIC EXAMPLE OF SIMILAR SOUNDING FLT NUMBERS; RADIO SPEECH AND ACFT OPERATING IN SAME AREA ON DIFFERENT ATC FREQS (INBND 120.5; OUTBND 118.1). CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR WAS F/O ON ACR X. SAYS PIC DID HAVE TELEPHONE REVIEW WITH ATC AFTER LNDG; AND LATER AFTER TAPES WERE PLAYED. CTLR HAD MEANT DSCNT CLRNC FOR COMPANY TRIP W WHO WAS WELL AHEAD OF X AND CLR OF DEP PATH OF Y. HOWEVER; CTLR USED WRONG CALL SIGN IN GIVING DSCNT CLRNC. RPTR SAYS HE THOUGHT IT UNUSUAL THAT THEY WOULD BE CLRED TO 3000' SO SOON; BUT DID NOT QUESTION CLRNC UNTIL THEY SAW ACR Y ON XING COURSE AT 5000'. RPTR DID READBACK DSCNT CLRNC; AND DID NOT HEAR COMPANY W ACKNOWLEDGE SAME CLRNC; BUT SAYS THEY MAY HAVE BLOCKED EACH OTHER IN READBACK. RPTR ACFT LEVELED OFF ABOUT 5800' TO PASS OVER Y. ACR Y HAD JUST BEEN POINTED OUT AS TFC FOR X. X AND Y WERE ON PARTYLINE; BUT CTLR NEVER DID ADVISE THAT THERE WERE SIMILAR CALL SIGNS ON FREQ.