How can you smoothly merge two .tex files into one without changing document style, etc

Here is a description of my problem: I have two documents, a main text and a supplementary text (both written in tex), which I would like to combine and submit to arXiv. arXiv requires that I submit a single .tex file. I attempted to submit a single file that combined two pdfs using pdfpages. My submission was eventually put on hold and they contacted me to say that using pdfpages was against the rules (this wasn't clear to me when I submitted it. In fact, I saw another SE post suggesting someone use this exact method to submit to arXiv). How can I combine these two tex files into one? Is there some command that can reset all parameters within a tex file? My hope is that there is some command like the following (with \BEGINNEWDOC acting like I suggested):

% document 1 (main text) \documentclass \title \begin \maketitle \section text. \end \BEGINNEWDOC % document 2 (supplementary text) \documentclass \title \begin \maketitle \section text. \end

Then this single file would be like creating the two documents separately then manually combining the pdfs. I am using Overleaf to edit the documents if that is relevant.

If you have two .tex files with the same preamble, you can just copy the contents between \begin and \end from the second file into the first one, can't you? I don't fully understand the problem here. Using \include or \input you will still need to submit the other files to be included. If you are not allowed to submit more than one file, I fear, there is no other way than to do some handiwork. But perhaps, I did not understand your case correctly.