Assuming they're QBs, I think that can be unilaterally done at any point because it would be one QB independently ending their coverage, while the other QBs maintain their independent election rights. That was my point above.
As to how it practically occurs, that will probably vary from plan to plan (or more realistically, TPA to TPA). The formal QB notice requirements in the COBRA regs surround the requirements upon divorce/legal separation, loss of dependent status, second qualifying events, disability determinations, etc. Notice of voluntarily dropping mid-maximum coverage period doesn't seem to be addressed.
Any reasonable requirement that the QB notify the plan of the change in coverage I would think suffice. Clearly there will have to be some form of notice for the plan to know that a) it's not just a premium shortfall situation, and b) which QBs are maintaining coverage.