This wiki page suggests there will be ("As of LyX 1.6") a module that would enable both footnotes as endnotes and displayed as usual footnotes, but I can't seem to find it: https://wiki.lyx.org/Tips/EndNotes : "In the 1.6 series, endnotes will be supported by two sorts of layout modules <https://wiki.lyx.org/LyX/Concepts>. One of them, *foottoend*, just implements this trick. Another provides direct support for endnotes, in addition to footnotes."
I'm using LyX 2.1.2 on Debian GNU/Linux. There is a workaround that keeps me changing the preamble: I get an error upon opening a document and trying to generate a PDF: "LaTeX Error: File 'file.ent' not found." Then If I uncomment a line in the preamble saying, "\let\footnote=\endnote", and regenerate the PDF, it compiles and displays (but the footnotes only exist as endnotes). Then, if I re-comment out that preamble line, and regenerate the PDF, the footnotes are in both places without error. And it continues to work until I close and reopen the document (when presumably file.ent is removed). Is there a module that I'm just not seeing, or another simple solution? Thanks, Luke