NMAC BTWN A MLG CLBING AND A SMA TWIN IN LEVEL FLT. ATC ADVISORY TO BOTH ACFT AND BOTH FLC HAD THE OTHER IN SIGHT.

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WE HAD DEPARTED NEWARK ARPT'S (EWR) RWY 4L WITH L TURNS TAKING US TO THE W OF ARPT. THE COPLT WAS FLYING; AND I WAS WORKING THE RADIO. AROUND 4000 FT THE COPLT ENGAGED THE AUTOPLT; AND ABOUT THIS TIME THE NEW YORK DEP CTLR CLRED US TO CLB TO 8000 FT AND INSTRUCTED US TO INTERCEPT THE COLTS NECK (COL) VOR 350 DEG RADIAL AND TRACK IT INBOUND. WE WERE THEN SWITCHED TO THE SECOND NEW YORK DEP CTLR. AFTER CHKING IN WITH THE CTLR; I VERIFIED OUR CLRNC TO INTERCEPT THE COL 350 DEG RADIAL AND TRACK IT INBOUND TO COL AND THEN DIRECT TO THE WHITE INTXN; WHICH WAS THE FIRST POINT ON OUR FLT PLAN. NOW ESTABLISHED ON THE 350 DEG RADIAL AND LEVEL AT 8000 FT; THE CTLR CALLED TFC AT 10 O'CLOCK AND; 8500 FT; HDG W A BEECHCRAFT BARON OPERATING VFR. BOTH THE COPLT AND MYSELF SPOTTED THE TFC; AND I ACKNOWLEDGED THIS TO THE CTLR. THE CTLR ALSO POINTED US OUT TO THE BARON; AND HE ACKNOWLEDGED HAVING US IN SIGHT. AT THIS TIME; THE CTLR ISSUED INSTRUCTIONS FOR US TO 'MAINTAIN VISUAL SEPARATION AND CLB TO 17000 FT.' I ACCEPTED THIS CLRNC AND ACKNOWLEDGED THIS TO THE CTLR; AND SOMEWHAT TO MY SURPRISE THE BARON STATED HE WOULD 'MAINTAIN VISUAL SEPARATION.' THE COPLT STARTED A CLB OUT OF 8000 FT USING THE PROFILE MODE OF THE FMS; AND IT NORMALLY TAKES SEVERAL SECONDS FOR THE PWR TO COME UP AND THE CLB TO START USING THIS METHOD. LOOKING AT THE BARON I COULD SEE HE WAS IN A SHALLOW L TURN; WHICH WAS TURNING HIM TOWARD THE SAME DIRECTION WE WERE HEADED; AND I COULD SEE WAS MUCH CLOSER THAN HE APPEARED WHEN WE FIRST SPOTTED HIM. I TOLD THE COPLT 'LET'S HOLD IT HERE; WE'LL BE CLR IN A MIN;' AND JUST AFTER I SAID THAT THE TCASII ANNOUNCED 'MONITOR VERT SPD.' USING THE VERT SPD SELECTOR WHEEL THE COPLT WAS TRYING TO STOP THE CLB. IN JUST A COUPLE OF SECONDS THE TCASII ANNOUNCED 'DSND.' I LOOKED UP AND COULD SEE THE BARON WAS ALMOST OVER US IN A STEEPER TURN THAN BEFORE ALMOST PARALLELING OUR HDG. AT THIS TIME I TOOK CTL OF THE ACFT FROM THE COPLT; DISCONNECTING THE AUTOPLT AND DSNDING TO 7900 FT. ABOUT 45 SECONDS LATER THE CTLR STATED WE WERE CLR OF TFC AND COULD CLB TO 17000 FT. PERCEPTIONS AND JUDGEMENT: AS AIRLINE PLTS WE ARE NORMALLY USED TO LOOKING AT LARGER ACFT; AND THE SCALE DIFFERENCE THREW OUR DISTANCE AND CLOSURE JUDGEMENT OFF. CREW COORD: I SHOULD HAVE MADE MY STATEMENT 'LET'S HOLD IT HERE; WE'LL BE CLR IN A MIN.' A DIRECTIVE RATHER THAN A SUGGESTION. WHEN I SAW THE COPLT USING THE VERT SPD WHEEL I SHOULD HAVE TAKEN CTL OF THE ACFT AT THAT TIME BECAUSE I KNEW THAT WAS A SLOW WAY OF STOPPING THE CLB.