> So far LaTex has been the only solution to allow binding shared sections

R, being a Turing-complete language, can do this. If R Markdown can do it then KeenWrite and pandoc+knitr support it. R can also do conditional includes. For Product A and B to share authentication steps, this could look like:

    `r#inc( '../common/authentication.Rmd' );`

Where TeX may resemble:

    \input ../common/authentication.tex

SGML, DocBook, AsciiDoc, RST, and other text formats have similar abilities.