I don't have that situation, but what I'd suggest is the following:
- First, use assignment groups with weights that are not dependent on the section: For example, have one assignment group for "Homework" that applies to homework for both sections, another for "Exams" that have the midterm and the final, and so on. It is important that every assignment group has at least one assignment for each 8-week section.
- Second, use the "assign to" policies in each quiz/assignment to assign the tasks only to the relevant section. So a "week 3" homework will be assigned only to the 16 week section and the part A section, while the final exam (in week 16) will only be assigned to group B and the 16 week session.
I think these two should be enough to get what you want.
This way, at the end of week 8, the part A section will have a grade for every assignment group that can be then used to calculate their final grade. Because none of the tasks for the second half is assigned to them, their grade will not be changed anymore.
For the part B section, they will not have missed any assignments yet, since they weren't assigned to any.
The 16-week section will have their current grade based on the past assignments, but that will be updated as new assignments are graded.