[NTG-context] How to add data from another PDF in index?

[Joel]

I am making a textbook and workbook set.
I have this custom index using this code, in both the textbook and workbook:
%
\defineregister[learnedshort][compress=yes]
\setupregister[learnedshort][style=sansbold, textstyle=slanted, n=2, pagenumber=yes, indicator=no]
%
\defineregister[learnedlong][compress=yes]
\setupregister[learnedlong][style=sansbold, textstyle=slanted, n=2, pagenumber=yes, indicator=no]
%
\define[1]\learnedis{%
    \txt{} \getvalue{#1}
    \learnedshort{#1}
    \learnedlong{#1}
}
\define[1]\learned{%
    \learnedshort{#1}
    \learnedlong{\getvalue{#1}}
}

\starttext

\learnedis{blah blah}

\input knuth

\learnedis{some more}


    \startchapter[title=Standards]
        \placelearnedshort
    \stopchapter
    
\stoptext


My question is, can I have references to the Workbook page numbers in the textbook? And visa-versa?

For instance, the Textbook index currently looks like this:
-------------
Standards
blah blah 1
some more 2
--------------
With the data in the Workbook it might look like this:
 Standards
blah blah 1, WB: 1, 3-8

some more 2, WB: 8-9
yet more WB: 10, 12

--------------
Is this possible to merge index information like this?
--Joel
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