TWO ACR-MLG; SAME COMPANY SIMILAR A/N CLIMBING SAME DIRECTION; WRONG ACFT ACKNOWLEDGED NEW ALT ASSIGNMENT RESULTING IN LESS THAN STANDARD SEPARATION BETWEEN ONE MLG AND AN MLT AT SAME ASSIGNED ALT.

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MLG X (ABCC) DEPARTED SFO ENRTE DEN CHKED ON FREQ AND WAS ASSIGNED CLB TO FL290. MLG Y (ABDC) DEPARTED SMF ENRTE TO SLC; CHKED ON FREQ AND WAS ASSIGNED FL270. BOTH ACFT WERE ADVISED OF THEIR SIMILAR SOUNDING CALL SIGNS AND TOLD TO USE CAUTION. MLT Z ENRTE FROM ABOUT THE FMG VORTAC DIRECT TO SHADD; MER (MLG X) CHKED ON LEVEL FL290. MLT Z AND MLG Y WERE HEAD ON TFC FOR EACH OTHER. DUE TO WX BUILD UPS OUT E ACFT WERE BEING HELD DOWN AT LOWER ALTS. WITH THIS IN MIND THE DECISION TO TAKE MLT Z DOWN A SOON AS WE COULD WAS REACHED. MLG Y (ABDC) WAS ISSUED 'AMENDED ALT CLB AND MAINTAIN FL250.' THE RESPONSE WAS; 'FL250 MLG (ABDC); ER (ABCC) EXCUSE ME (ABCC).' THE CORRECTION TO ACFT IDENT WAS MISSED BY THE CTLRS. MLT Z WAS DSNDED TO FL260. MLG Y (ABDC) WAS OBSERVED PASSING THROUGH FL255; PUTTING HIM IN JEOPARDY WITH MLT Z. MLG Y (ABDC) WAS REISSUED FL250 BUT DID NOT RESPOND. MLT Z WAS TOLD TO MAINTAIN PRESENT ALT. Z RESPONDED; 'LEVEL FL265.' MLG Y (ABDC) WAS AGAIN ISSUED 'DSND AND MAINTAIN FL250 IMMEDIATELY;' BUT AGAIN THE ACFT DID NOT RESPOND. MLG Y (ABDC) WAS THEN ISSUED A TFC ALERT AND ONLY THEN RESPONDED. WHILE I FEEL THERE WERE MANY CAUSAL FACTORS SIMILAR SOUNDING CALL SIGNS WAS THE BIGGEST FACTOR. MLG X (ABCC) RESPONDED TO MLG Y (ABDC) CLRNC WITH MLG Y (ABDC) CALL SIGN LEADING THE CTLR TO BELIEVE THAT THE CORRECT ACFT HAD RECEIVED THE AMENDED ALT. ONCE THE ERROR WAS SPOTTED BY THE CTLRS; WE WERE UNABLE TO CORRECT IT AS MLG Y (ABDC) COULD NO LONGER BE REACHED ON FREQ. WE MISSED THE CONFUSING READBACK AND SO DID MLG Y. MLG Y DID NOT MAINTAIN DILIGENCE TO THE RADIOS AND THE LOSS OF SEP OCCURRED. SUPPLEMENTAL INFO FROM ACN 89717: AT APPROX 26500' A LARGE MIL ACFT WENT FROM LEFT TO RIGHT JUST ABOVE OUR ALT. CENTER LATER TOLD US OVER THE PHONE THAT THEY ATTEMPTED TO GIVE US FL250 BUT ANOTHER ACFT WITH A VERY CLOSE CALL SIGN ACCEPTED THE CLRNC INSTEAD OF US. I FEEL THAT THERE MUST BE A CHANGE TO CLARIFY ACFT CALL SIGNS WHEN THEY ARE SIMILAR. SUPPLEMENTAL INFO FROM ACN 89812: I SAW THE OTHER ACFT AT MY 12 O'CLOCK (FILLING UP MY WINDSCREEN) PASSING 26500'. I HAD NO TIME FOR EVASIVE ACTION EXCEPT FREEZING THE CLB BEFORE THE OTHER ACFT PASSED LEFT TO RIGHT AND ABOVE. AFTER THE INCIDENT (PHONE CONVERSATION WITH ZOA SUPVR); I CALLED CENTER AFTER THEY LISTENED TO THE TAPE. 2 ACFT WITH SIMILAR CALL SIGNS WERE ON THE SAME FREQ. IN THE CONFUSION OVER WHO WAS TO DO WHAT; THE CTLR TOLD THE OTHER ACFT TO LEVEL OFF AT FL250; WHEN IN FACT WE WERE THE ACFT HE ACTUALLY WANTED TO LEVEL OFF AT FL250. THE OTHER ACFT LEVELED AT FL250 AND OF COURSE WE CONTINUED OUR CLB. WE WERE BEING VERY EMPHATIC OVER OUR IDENT WHEN ANSWERING AND TALKING TO ZOA; BUT STILL THE PROB AROSE. WE MUST DEVELOP/USE A MORE FOOL PROOF/EMPHATIC/NO CONFUSION CALL SIGN SYS WHEN ACFT OF SIMILAR CALL SINGS ARE IN THE SAME AIRSPACE; ESPECIALLY ON THE SAME FREQ. EITHER ADD A SECONDARY TERM SUCH AS 'ACR (ABCC) BLUE;' AND/OR 'ACR (ABDC) RED;' WHICH WAS THE CASE THERE (ACR [ABCC] AND [ABDC]) IN ORDER TO LESSEN THE CONFUSION OVER WHO SHOULD RESPOND. THIS PROB IS EVEN MORE CRITICAL IN THE VICINITY OF HUBS BECAUSE OF THE NUMBER OF ACFT WITH SIMILAR CALL SIGNS (SAME COMPANY). MAYBE EACH CAPT SHOULD SELECT A PERSONAL CALL SIGN TO ADD TO HIS COMPANY FLT NUMBER. THE MIL DOES THIS. IT CERTAINLY GOES A LONG WAY IN IDENTING YOUR SPECIFIC FLT.