LEGISLATORS yesterday more than doubled the allowance for jurors and witnesses in the first adjustments in a decade. The daily allowance for jurors jumped from $90 to $240 while the allowance for expert witnesses leaped from $200 to $1,400. The amendments - which will cost an extra $4.3 million a year - will take effect by the end of January. The allowance for ordinary witnesses in criminal cases and coroners' inquiries will rise to $240 a day. In December 1991, legislators were invited to approve a proposal to revise the rates based on inflation, but the proposal was dropped.