Thank you Peter for your advice and your footnote script!
Thanks Ariel for answering. I downloaded the trial version. I do not know why the script only works for texts in one column. The script converts the two columns into a single one and if later I convert the main text into two columns .. there is no synchronization with the notes. Perhaps the problem is that the base text has many long footnotes.The footnotes are 80% the size of the sheet of paper. As if it were a long text comment.
Sorry I only saw this now.
The trick with Footwork is to apply an object style to the first text frame of the footnotes. In that object style, set it to have 2 columns (or however many you want).
Now when you run Footwork, it will use that object style for all the footnote text frames it creates. You can do the same for the main-text text frames (that is, create an object style for the first one and Footwork will apply it to all new frames it creates).
Many long footnotes are fine, and Footwork has various features to help layout documents with long footnotes, notably:
(1) A synchronization slider which lets you define how much of a long footnote should be typeset before more main text is typeset. This creates much better looking pages than InDesign's default (and only) setting, which is to lay out all the footnote before moving on to the next line of main text, which creates a very unbalanced page. In fact, I don't know of any other software that gives the user this elementary level of control.
(2) The ability to allow footnotes to begin on a page following the footnote indicate in the main text. With long footnotes, there is often a problem that if 2 footnotes appear close together, a large white gap is created at the bottom of the page. With Footwork, you can use the option that allows a footwork to appear on the next page. Not ideal, but better that a lot of white gaps at the bottom of pages. (Of course, the best solution is for the editor to move such long footnotes into a separate appendix. If you're lucky...)