PHH699 wrote:
must the accompanying passenger be on
the same exact journey as me (the network card holder)? (N.B. I will be with
the "accompanying passenger" for the entirety of their journey, but the
"accompanying passenger" will not be with me for the entirety of my
journey).

That's fine. As long as both parties are travelling with the railcard,
then you'll be fine. If your second ticketholder were stopped whilst
alone, they would not be able to produce the railcard, and therefore
would not have a valid ticket. Of you then head home alone, you would
have a valid ticket, backed up by the railcard.