@evan Twitter / X have public replies from B visible to A's followers as they are open. But, B's followers can see the response, which is where things get to be problematic.
I wasn't intending to say only B's followers saw the reply, but that they could see the response to a private account.
Marketers, stalkers, and worse have easy pickings in that model.
What @dahukanna lays out in the venn diagram is the good approach.