For me, the issue is not document conversion and is content de-duplication. Product A and Product B manual will share number of sections. Any spelling or grammar error must be updated in Product A and B manuals. They man share the same additions.
So far LaTex has been the only solution to allow binding shared sections into multiple document builds. These are for internal and customer facing information. Only binding duplication is in the revision overview sections pertaining to each document build.
> 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:
Where TeX may resemble: SGML, DocBook, AsciiDoc, RST, and other text formats have similar abilities.