Model: A country map consisting of state objects (take, for example, the USA with its states like Florida). Each state is bounded by a closed multipoint line. The boundary of a state with a neighbour state is a (normally open) part of its boundary that coincides with part of the neighbour's boundary. A boundary node point may coincide with node point of multiple neighbours.
Problem: I hear that one of my states is, in reality, two states, e.g. it is not Carolina but North Carolina and South Carolina. Their common boundary is a multipoint line somewhere from East to West and is complicated to draw, so I draw a Southern boundary for Noth Carolina and copy that as Northern boundary for South Carolina. My (wrong) Carolina will be split in two parts by selecting two boundary points, one in the West, the other in the east. Then the boundaries are closed by using the prepared boundary lines.
I cannot use (Premium) Knife because you cannot point to a boundary point: (almost) all boundary points coincide with the boundary point of a neighbour!
I have the idea that I should use "Shaping objects", "Break a path" and "Extract a broken path from an object" but I cannot find a way to reach my goal.
What is a useful strategy?