\usebodyfont  [stix]
 \setupbodyfont[modern]

 \setupmathematics[stylealternative=calligraphic]
 
 \unexpanded\def\CScript#1#2{\start\switchtobodyfont[stixtwo] \m{{\mathscript{#1}}_{#2}}\stop}

 \unexpanded\def\cscript#1{\start\switchtobodyfont[stixtwo]\m{{\mathscript{#1}}}\stop}

\starttext
\cscript{P}
\startformula
  \cscript{P}
\stopformula
\stoptext