I have had such an experience. One road was the primary access and two others were separate offshoots with differing maintenance issues. Those two road associations contributed to the primary association. The relationship lasted decades, was very informal and reasonably harmonious until dissolving in acrimony over major road improvement costs to the access road. Ultimately, the municipality, citing health and safety issues, stepped in, condemned the roads, improved them to municipal standards and assessed all owners for the expense. This was in NJ which did not have a private road statute. I suspect municipal takeover is not likely in most areas in Maine but given human nature, dissolving into acrimony is reasonably foreseeable.
Our private roadway here in Maine has the exact same setup but is handled differently - there is one gravel access road that everyone uses and on which there are no residential parcels (runs through land trust), one gravel road offshoot and a separate paved road offshoot. Everyone contributes equally for normal maintenance including winter maintenance over the entire roadway system. Each of the offshoots pay for extraordinary maintenance related their sections. Everything is administered as one informal association. This system has functioned successfully for over 40 years.
Understanding how owner attitudes can change, our group is considering a statutory association format because it provides a formal structure with recognized benefits yet permits a great deal of self-determination in drafting bylaws. The group intends to remain one association for operational, assessment and collection efficiency.
Perhaps in your situation a similar approach is warranted: decide what is logical, including what is a reasonable and fair assessment scenario, and unite under one association for efficiency.