An Index requires secondary work to make hyperlinks so that is a problematic method.

The earlier suggestions of two separate TOCs is going to work easily enough if you can figure out a method. Let's say you need three styles for the headings and to make it obvious lets call them "Head Both", "Head Infrastructure", "Head Parks"

Then for the TOC fields you can set up text and fields like this
Infrastructure
{TOC \t "Head Both,1,Head Infrastructure,1" \h}

Parks
{TOC \t "Head Both,1,Head Parks,1" \h}

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Andrew Lockton
Chrysalis Design, Melbourne Australia