DEP CTLR FAILED TO MAINTAIN LEGAL SEPARATION BTWN A CESSNA 210 AND A PIPER PA31 CLBING OUT FROM 2 DIFFERENT ARPTS BUT PARALLELING EACH OTHER ON THE SAME FLT PLAN RTE.

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THE CENTURION DEPARTED NEWPORT ARPT AND I ISSUED CLB TO 10000 FT. 2 OR 3 MINS LATER; THE PA31 DEPARTED PROVIDENCE FLT PLANNED VIA THE SAME RTE AND ALT. I ISSUED CLB TO 8000 FT AND PUT BOTH ACFT ON PARALLEL HEADINGS. MY WORKLOAD WAS LIGHT AND I BELIEVED I HAD PLENTY OF TIME TO MONITOR THESE 2 ACFT. APPARENTLY THE SCOPE WAS SET UP ON A SLIGHTLY DIFFERENT RANGE THAN WHAT I WOULD BE ACCUSTOMED TO AND I BELIEVED I HAD AT LEAST 5 MI BTWN THE 2 ACFT WHILE THEY WERE ON PARALLEL HEADINGS. I SAW THE RADAR PLOT LATER AND THERE WAS ACTUALLY ONLY 3.5 MI BTWN THEM. AS THE ACFT CLBED NWBOUND AND BELIEVING I HAD A CUSHION ON 2 MI I INSTRUCTED THE CENTURION TO RESUME HIS OWN NAV DIRECT TO PUT VOR (A COURSE THAT WOULD CONVERGE WITH THE HEADING OF THE PA31) LEAVING THE ASSIGNED ALT OF THE NAVAJO (8000 FT). MEANWHILE I STARTED TO RECEIVE A HDOF OF A SBOUND PA31 AT 8000 FT THAT WOULD BE A CONFLICTION WITH THE PA31 I WAS ALREADY WORKING; SO I TOOK A MOMENT OR TWO TO DECIDE HOW I WOULD HANDLE THE SIT. DURING THIS TIME; THE CENTURION LEFT 8000 FT AND TURNED TOWARD PUT AND THE PA31. IF THERE REALLY HAD BEEN 5 MI BTWN THEM TO START EVERYTHING WOULD HAVE BEEN FINE; BUT AS THERE WAS ONLY 3.5 MI; I LOST STANDARD SEPARATION VERY SHORTLY AFTER THE C210 BEGIN HIS TURN TOWARD PUT. WHAT I DID WRONG: ISSUED A TURN LEAVING THE ASSIGNED ALT OF THE OTHER ACFT RATHER THAN ONE 1000 FT ABOVE IT; NEGLECTING TO MONITOR BOTH ACFT TO ENSURE THE SIT WAS DEVELOPING AS INTENDED; ALLOWED A POTENTIAL CONFLICT TO DISTRACT ME FROM AN IMMINENT ONE; NEGLECTING TO SET UP THE SCOPE ACCORDING TO MY NORMAL PRACTICE AND THEREBY CREATING CONFUSION AS TO SCALE; I FORGOT THAT I COULD HAVE TURNED THE CENTURION TOWARD BAF (THE NEXT FIX OUT OF MY AIRSPACE PER LOA WITH Y90 TRACON) THEREBY ESTABLISHING AND MAINTAINING LATERAL SEPARATION WITH THE PA31. I SETTLED ON A COURSE OF ACTION AND THEN CLOSED MY MIND TO ALTERNATIVES THAT WOULD HAVE VOIDED PROBS.