SMT HAS NMAC WITH LTT ON DSCNT INTO BOS.

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ENRTE TO BOS ON A VFR FLT PLAN AT 5500 FT. FLT X A DO-228 ENTERED MHT AIRSPACE AND ASKED FOR RADAR ADVISORIES. MHT ATC WAS WORKING A SAAB 340 FLT Y IFR ENRTE TO BOS (DO-228 WAS ABOUT 3-5 MI AHEAD OF THE SAAB). MHT ATC ADVISED THE SAAB CREW THAT THEY HAD DO-228 TFC AT 12 O'CLOCK; 5 MI. SAAB CREW ACKNOWLEDGED THAT THEY WERE LOOKING. MHT ATC ASKED FLT X WHAT THE HDG WAS TO BOS; I STATED IT WAS 190 DEGS. MHT ATC ADVISED THE SAAB CREW OF MY HDG. I AM NOT SURE WHAT THEIR RESPONSE WAS TO ATC. MHT ATC THEN HANDED US OVER TO ZBW (ABOUT 24 DME FROM BOS). BOS ATC CLRED FLT X INTO THE CLASS B AIRSPACE. DSND AND MAINTAIN 4500 FT. TURN R TO A HDG OF 240 DEGS; VECTORS TO FINAL. MOMENTS LATER AT 5000 FT ON THE 240 DEG HDG MY FO LOOKED OUT THE R WINDOW AND SAW A SAAB 340 PASS JUST UNDER NEATH US IN A NOSE-DOWN ATTITUDE. THE SAAB CREW CHKED IN THE ZBW AND STATED THAT THEY JUST HAD TO MAKE AN EMER MANEUVER TO AVOID HITTING A DORNIER. ZBW RESPONSE WAS ROGER! THE SAAB CREW THEN ASKED BOS WHAT HAPPENED BACK THERE? WAS THE DO- 228 ROAMING AROUND OUT THERE AND WHOSE AIRSPACE WAS HE IN? ZBW SAID THAT HE WASN'T SURE WHOSE AIRSPACE (BOS OR MHT) THE DO-228 WAS IN (IT WAS AROUND THE 20 DME). THE SAAB CREW THEN STATED THAT IT WAS CLOSE. TO MAKE A SIMPLE STORY; WE CROSSED EACH OTHER'S PATH AT THE SAME ALT. THIS COULD HAVE BEEN AVOIDED IN A COUPLE OF WAYS: MHT ATC KNEW HE HAD 2 ACFT ON THE SAME HDG LNDG AT THE SAME ARPT. WHEN HE HANDED THE DO-228 OFF TO ZBW; HE SHOULD HAVE NOTIFIED HIM (BOS) ABOUT THE SAAB WHICH IS BEHIND THE DO-228 THAT IS ON THE SAME HDG AND IS MUCH FASTER. HOW ABOUT A HDG CHANGE OR A SPD REDUCTION FOR THE SAAB? THE SABB CREW EITHER NEVER SAW US OR FAILED TO MAINTAIN VISUAL CONTACT WITH US (DO-228). THIS HAPPENED IN A TRANSITION PERIOD (HDOF). I FEEL THAT THE SAAB CREW WAS UNPROFESSIONAL OVER THE RADIO. HE MADE IT SOUND LIKE THE DO-228 WAS ROAMING AROUND OUT THERE LOST; WHERE IN FACT I HAD A CLRNC TO OPERATE IN THE BOS CLASS B AIRSPACE WITH INSTRUCTION FROM ATC.