Create TOC from document with two text flows

With paragraph numbering you could add a counter at the beginning of the paragraph. In the left column let it start with 1 and in the right column add a character like # before the counter. And after the counter add a unique character like &.

This would be something like:

<n>< >& for the first column

#< ><n>@ for the second column

Then update your TOC. This way the TOC headings of the two columns are not mixed.

Then you can cut the TOC headings of the second column easily and paste them into the second TOC column

And then delete all this special numbering with regular expressions.

Not elegant, but not too many extra steps.

I can't think of a way to do this with stock FrameMaker unless you have two separate books and TOCs and then combine them into a two-column document with text insets. Otherwise, we can probably devise a clever scripting solution.