A402 ENTERING TFC PATTERN AT MVY AT 1200 FT EXPERIENCED NMAC WITH C402 DEPARTING PATTERN.

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DURING ARR AT MVY; TWR INSTRUCTED ME TO FOLLOW A CHEROKEE AT MY 9 O'CLOCK POS. WITH A GND SPD OF 230 KTS. I REQUESTED L BASE ENTRY (ARRIVING FROM THE N). TWR DENIED THE REQUEST AND INSTRUCTED ME TO RPT MIDFIELD DOWNWIND TO RWY 6. FROM THE CURRENT POS IT REQUIRED ME TO TURN L; GO AROUND THE CHEROKEE. AS I COMMENCED THE TURN TO RE-ENTER THE DOWNWIND; MY ACFT AND ANOTHER COMPANY ACFT THAT DEPARTED RWY 6 AND TURNED NBOUND ON COURSE WERE IN A COMPROMISING POS THAT REQUIRED BOTH PLTS TO MAKE EVASIVE MANEUVERS TO AVOID COLLISION. SUPPLEMENTAL INFO FROM ACN 631468: DURING ROLLOUT; TWR ADVISES OF TFC ENTERING FROM DIFFERENT AREAS. IN GENERAL; NO ALTS OR CLOCK POS. FLEW RWY HDG TO 700 FT; INITIATED L-HAND TURN TO 340 DEGS. THROUGH 1000 FT I TURNED OFF LIGHTS AND PUMPS AND ACCELERATED TO 130 KTS PER COMPANY PROFILE. THROUGH 1300 FT MSL; I NOTICE COMPANY AT 12 O'CLOCK POS ON A COLLISION COURSE. COMPANY TFC MADE CONSTANT ALT L BANK. SO I MADE HARD L BANK WITH DSCNT TO PUT SOME DISTANCE IN ALT. I CALLED TWR UPON ARR N BOSTON. CTLR SAID 'WE TOLD THE COMPANY TFC TO ENTER MIDFIELD L DOWNWIND FOR RWY 6. HE ASKED FOR L BASE. TWR SAID TO 'FOLLOW CHEROKEE ON DOWNWIND; OUR RADAR SHOWS HIM ON DOWNWIND.' HE WAS A LOT MORE TO THE NE THAN WE THOUGHT. NO TA'S REGARDING ALT OR CLOCK POS WERE GIVEN ONCE AIRBORNE.