The extent to which one ticket can be "combined" with another when the validity of one of the tickets extends either side of the other, is open to debate, I admit. But just as there is no statement in the NRCoT or Advance ticket T&Cs that "stopping short on an Advance ticket is strictly forbidden where it results in a financial advantage", the very phrase "stopping short" is not used.
Both the NRCoT and the Advance ticket T&Cs use the phrase "end your journey" (NRCoT 16.4 and 16.5), with the NRCoT expressly stating that "a journey" can consist of "a combination of two or more tickets" (NRCoT 14.1). Therefore, a passenger on a Euston to Lancaster ticket has not "stopped short" when alighting at Preston on a Wigan to Preston ticket if the two tickets "combine" (which I suggest they do, despite their overlapping validity*) and thus the passenger ends 'their journey' using the Wigan to Preston ticket.
*WARNING: Conjecture!