OPERROR WHEN B727 RPTS QUARTER MI FINAL AND IS INSTRUCTED TO GAR WITH A SIDESTEP TO THE R DUE TO A B757 ON TKOF ROLL ON THE SAME RWY. BOTH ACFT ENTERED THE CLOUDS ABOUT THE SAME TIME. THE B727 WAS SUBSEQUENTLY TURNED TO A DIVERGENT HDG. RO DETERMINED THIS WAS NOT AN OPERROR. CTLR WAS DECERTIFIED 3 DAYS LATER.

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TWR CLRED A B727 TO LAND ON RWY 16R. ABOUT 7 MINS LATER; A B757 WAS CLRED TO TKOF RWY HDG ON RWY 16R. NO EXCHANGE OF TFC TO EITHER ACFT. APPROX 1 MIN LATER; THE B727 RPTED '1/4 MI FINAL.' TWR SUBSEQUENTLY INSTRUCTED THE B727 TO 'GAR SIDESTEP TO THE R.' TWR THEN TURNED THE B727 R TO 180 DEGS. BOTH ACFT DISAPPEARED INTO THE CLOUDS. AS THE RADAR CTLR; I OBSERVED 2 TARGET DEPARTING RWY 16R. THE 2 ACFT WERE SEPARATED BY LESS THAN 1 MI LATERALLY AND LESS THAN 500 FT VERTLY. THE CAUSE WAS TWR CTLR NOT USING GOOD JUDGEMENT BTWN ARRIVING AND DEPARTING ACFT. CORRECTIVE ACTION: ACCORDING TO THE REGION HEADQUARTERS; THERE WAS NO OFFICIAL SYS DEV. HEADQUARTERS INDICATED THIS OCCURS FREQUENTLY; BUT THERE WAS NOT A LOSS OF SEPARATION. LOCALLY; THE CTLR WAS DECERTIFIED. HE WILL GET REMEDIAL TRAINING AND EVENTUALLY RECERTIFIED. SUPPLEMENTAL INFO FROM ACN 365369: I LOOKED UP TO OBSERVE A B727 EXECUTING A CLBOUT ON SHORT FINAL TO AVOID A B757 JUST STARTING A TKOF ROLL. THE B757 ENTERED THE CLOUDS AT ABOUT THE SAME TIME AS THE B727; AND THE B727 WAS TURNED 25 DEGS FROM RWY HDG.