I am exploring the feasibility of presenting two interacting PDF files side
by side. Thus far my experiments with \usereferences have managed only to
produce  a lua error:

.../ConTeXt/tex/texmf-context/tex/context/base/strc-ref.lua:1394: attempt
to call field 'reference' (a nil value)

stack traceback:

.../ConTeXt/tex/texmf-context/tex/context/base/strc-ref.lua:1394: in
function 'identify_outer'

.../ConTeXt/tex/texmf-context/tex/context/base/strc-ref.lua:1620: in
function 'identify'

.../ConTeXt/tex/texmf-context/tex/context/base/strc-ref.lua:1645: in
function 'valid'

.../ConTeXt/tex/texmf-context/tex/context/base/strc-ref.lua:1667: in
function 'doifelsereference'

 [\directlua]:1: in main chunk

The setup--

%testA.tex
\starttext

text text.\reference[firstpage]{}

text text.\footnote[fnote]{A note.}

\stoptext

%LinkTest.tex
\setupinteraction[state=start]

\useexternaldocument[testA.tex]
\usereferences[testA.tex]

\starttext

\at{page}[testA.tex::firstpage]

\in{note}[testA.tex::fnote]

\stoptext