That you can't concat two word files together in 2018 is ridiculous.
Of course you can!
# cat word1.doc word2.doc > word3.doc
Sounds ludicrous? So does your statement.
Setting aside the hyperbole, I believe it would be a conceptual nightmare to start defining concat of two docs. For simple cases, feel free to make copy pasta :-)
Just append the pages of document 2 to the end of document 1. Then the user can decide whether to remove the page break introduced by it. I have this in my env for doing this with PDF:
$ cat =concatpdf
if [ "$1" = "to" ]; then
gs -q -sPAPERSIZE=letter -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -sOutputFile=$*
echo Usage: concatpdf to out.pdf in.pdf in2.pdf in3.pdf
$ pandoc file1.md file2.md -o file.docx
It isn't difficult. TeX and LaTeX have been doing it since the 70s and 80s respectively.