\defineprocessor[twolines][command=\MyStyle]

\unexpanded\def\MyStyle#1%
  {\setbox\scratchbox\hbox{#1}%
   \scratchdimen\dimexpr\hsize-\leftskip-\hangindent\relax
   \ifdim\wd\scratchbox>\scratchdimen
     \aftergroup\hangafter
     \aftergroup\plustwo
     \aftergroup\break
   \else\ifdim\wd\scratchbox>2\scratchdimen
     \aftergroup\hangafter
     \aftergroup\plustwo
     \aftergroup\break
   \fi\fi
   \unhbox\scratchbox}

\setupregister
  [balance=no]

\setupregister
  [index:1]
  [textstyle=\MyStyle]

\starttext
    \index{primary+secondary+tertiary}
    \index{twolines->primary long primary long primary long primary long primary long}
    \index{twolines->primary longer primary longer primary longer primary longer primary longer}
    \input{tufte}
    \completeindex
\stoptext
