I'm reading it the same way as @Doc Maik, so if correct, this is yet another case for InDesign to allow footnote cross-references. That would allow you to add the footnote to the English side, and then cross-refererence the footnote reference number on the Greek side, so that both reference numbers are pointing to the same footnote. If you add a footnote in the middle, all the footnote numbers and cross-references to those numbers will update.
This feature is still not available in InDesign, but has been requested:
https://indesign.uservoice.com/forums/601021-adobe-indesign-feature-requests/suggestions/31730974-enhanced-cross-references-footnotes
In the meantime, @istos22981053eke3, it looks like this plug-in can handle it. I haven't tested it.
https://www.dtptools.com/help.asp?id=40
~Barb