LTSS BTWN 2 SMA ACFT DEPARTING SAME ARPT IN IMC WX. OPERATIONAL ERROR.

Narrative

I DEPARTED SMA X IFR OFF RWY 11 WITH A 220 DEG HDG ASSIGNED. APPROX 3 MINS LATER; SMA Y CALLED READY HDG BASICALLY THE SAME GENERAL DIRECTION (SWBOUND). SCANNING THE BRITE; I NOTICED SMA X 2 NM S OF BED ON WHAT APPEARED TO BE A 220 DEG HDG. CONSIDERING THAT X TYPE IS FASTER THAN Y TYPE I; WHO WAS NOW TRAINING A DEVELOPMENTAL; INSTRUCTED MY STUDENT TO DEPART ACFT Y AND ASSIGN A 220 DEG HDG ALSO. (220 DEG IS THE MAX SW HDG I CAN ASSIGN ACFT OFF RWY 11 BY LETTER OF AGREEMENT). WHAT HAPPENED NEXT; I AM UNSURE OF BECAUSE OF 2 REASONS: I WAS TALKING IN DEPTH TO THE TRAINEE EXPLAINING INST APCH PROCS. I ALLOWED MY ATTN TO BE DIVERTED FOR TOO LONG OF A PERIOD OF TIME. RADAR COVERAGE S AND SW OF HANSCOM FIELD IS NONEXISTENT AFTER 2 NM (DUE TO HIGH BUILDINGS BLOCKING RADAR PULSES). APPARENTLY Y OVERTOOK X AND DEP CTL WAS FORCED TO TURN 1 OF THE ACFT TO AVERT A CONFLICT. IN THE FUTURE; NOT ASSUMING THAT AN ACFT (OF SAME CATEGORY) WILL BE FASTER AND NOT GETTING DISTR COULD ALLEVIATE A SITUATION LIKE THIS OF HAPPENING AGAIN.