MLG MISSED NEW ALT ASSIGNMENT AND ARTCC CTLR HAD LESS THAN STANDARD SEPARATION BETWEEN MLG AND SMT.

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AN ACR MLG X DEPARTED TUS; HEADED NWBND OVER GBN CLBING TO FL310. THE SECOND ACFT; A CORPORATE SMT Y; HEADED SE TO GBN THEN TUS. WHEN BOTH ACFT WERE ABOUT 30 APART AND THE ACR WAS STILL CLBING OUT OF FL250; THE CTLR TOLD THE ACR TO MAINTAIN FL280. THE CTLR THEN HEARD A REPLY. WHEN THE TFC WAS AT 10 MI THE CTLR CALLED TFC FOR BOTH THE MLG X AND THE SMT Y WITH THE MLG X STILL CLBING OUT OF FL270 TO FL280. THE ACR THEN QUESTIONED HIS ALT ABOUT 30 SECS LATER BY SAYING HE WAS ASSIGNED AT FL310. THE CTLR REPLIED THAT HE HAD BEEN GIVEN A SECOND CLRNC TO FL280. THE ACR SAID THAT THEY HADN'T RECEIVED ANY CLRNC. THE CTLR DSNDED THE ACR 500' TO FL280 AND TURNED THE SMT Y AWAY FROM THE MLG X. A FEW MINS LATER ANOTHER ACR CALLED AND SAID HE HAD CHKED ON WITH CENTER TWICE BEFORE AND NEITHER CALL WAS HEARD. THE CTLR XMISSION WAS BLOCKED BY THE SECOND ACR CHKING ON. THE CTLR DIDN'T GET A COMPLETE READBACK FROM THE MLG X. THE CTLR SHOULD GET A COMPLETE READBACK FROM THE ACFT WHICH INCLUDES: CALL SIGN AND CLRNC. IF HE/SHE DOESN'T GET A COMPLETE READBACK; GET ON THE ACFT FOR ONE. ALSO; DUE TO A PROB WITH FREQ'S CLARITY; THE CTLR MISTOOK THE SECOND ACR'S CHK ON AS THE FIRST ACR READBACK. HAD THE FREQS BEEN BETTER THE CTLR MIGHT HAVE CAUGHT THE ERROR BEFORE IT OCCURRED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR GAVE EXPERIENCE LEVEL; ONLY BEEN FPL FOR APPROX 6 MONTHS. PROX OF TFC WAS 4.2 NM AND 500'. RPTR WAS WORKING A COMBINED POS OPERATION AND NOT USED TO HIGH ALT RADIO SOUNDS.