Interchange

If the line doesn't interchange cars with the rest of the rail system, then official marks aren't necessarily required.

Your narrow gauge line might transload freight, but given the guage difference probably can't exchange cars. (Although there have been examples of narrow gauge lines that re-trucked standard gauge cars for travel (see Canadian National's Newfoundland lines).

Multiple lines?

If multiple 2-footers exchanged cars, would the reporting marks become necessary?  In my fictitious history, there would have been interchange between the WWF and the SR&RL before they officially merged

Interchange

Interchange is not just cars.  Interchange is rates between common carriers.  If the line can be quoted in a rate and receive a division of the revenue, then it needs to have reporting marks.

If the railroads quote a rate WWF-Farmington-BM-Boston-CR-Chicago-CNW-Fremont-UP, the WWF would be part of the route and would require reporting marks, even though its cars never left its lines.

If you look at older ORER's, the narrow gauge lines were all listed.

solved

Thanks for the answers.  It looks like I'll use WWF for the Wiscasset, Watervill, and Farmington, SRRL for the Sandy River and Rangely Lakes, and WWRL for the merger, which is officially "Wiscasset, Waterville, and Rangely Lakes," though it's known as "The Great North Central" unofficially.  Later on, when the name officially becomes "Great North Central," GNC will be the new reporting mark.  All of these are not in use, according to Wikipedia.