That is a good question. When we come across shared sewer lines in the course of a home sale/purchase the "rule" of action is to separate, but I am not aware of any legalities that are involved and you would need to contact your local municipality or a lawyer for final clarification.
It is my understanding that the City of Portland is trying to flag and separate these lines, but the sales transaction can certainly go through. The buyer who inherits the party line will likely start receiving letters from the city to unhook the line, but I do not know how the city would force this if the sewer is otherwise working properly. They may become annoying:)