A B727 FLC ON A VISUAL SEPARATION CLB SUFFERS A LOSS OF SEPARATION BEHIND A BE76 WHEN THEY ENTER CLOUDS AT 1600 FT.

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DEPS OFF OF RWY 31 AT LGA. THE BEECH DUCHESS TOOK OFF AND WAS GIVEN A 340 DEG HDG. WE WERE CLRED INTO POS AND HOLD ON RWY 31 AND ASKED IF WE COULD MAINTAIN VISUAL SEPARATION. WE INDICATED THAT WE COULD BUT SINCE THE CEILING WAS ABOUT 1600 FT (THIS WAS LOWER THAN ATIS WAS RPTING BUT WE HAD JUST ARRIVED AND QUICK TURNED SO WE KNEW IT WAS ABOUT 1600 FT) WE WOULD ONLY BE ABLE TO DO IT FOR A SHORT TIME UNTIL WE GOT INTO THE CLOUDS SO WE DECLINED THE CLRNC. TWR THEN SAID WE ONLY NEEDED TO MAINTAIN VISUAL SEPARATION UNTIL WE WERE ESTABLISHED ON OUR (360 DEGS) HDG AND THEN SINCE THE HEADINGS WERE 'DIVERGENT' WE NO LONGER HAD TO MAINTAIN VISUAL. WE ACCEPTED THE CLRNC AND WERE CLRED FOR TKOF. WE KEPT VISUAL ON THE BE76 UNTIL ON THE 360 DEG HDG AND IN THE CLOUDS. WE WERE HANDED OFF TO DEP AND DEP ASKED IF WE STILL HAD THE BEECH IN SIGHT. WE DID NOT (WE WERE IMC; ETC). HE THEN ASKED HOW WE WERE MAINTAINING VISUAL. WE SAID THE TWR CTLR SAID WE ONLY HAD TO MAINTAIN VISUAL UNTIL ON THE 360 DEG HDG AND THE DEP CTLR SAID THIS WAS INCORRECT; YELLED AT US; AND GAVE US A NEW HEADING. I THOUGHT THAT THE TWR AND DEP WERE BETTER COORDINATED -- I GUESS NOT. I'M NOW FINDING THAT ANYTHING THAT INCREASES DEP AND ARR RATES SHOULD BE VIEWED WITH GREAT FEAR AND DECLINED (IE; VISUAL DEP; LAHSO; SIMULTANEOUS VISUALS AND VISUAL APCHS). ALL OF THESE PROCS SEEM NOT COMPLETELY THOUGHT OUT AND THE PLTS SEEM TO ALWAYS TAKE THE BLAME.