"Both" at least makes the full conversation visible to all A's followers, but it has most of the same problems as sharing with B's followers. It sends them a part of a conversation without context, but also violates A's privacy.
For "Other", a lot of people replied with "the intersection of A's followers and B's followers". This makes replies to replies to replies less and less visible to participants, until practically no one can see what's being said. It's terrible for conversations.
@evan it nonetheless sounds right to me when the conversation is initiated as followers-only; a choice for a semi-private audience. Each conversation thread should narrow in to smaller audience, even if that means eventually parts of it are essentially mention-only.
@novelgazer the original post and the replies don't have to be isomorphic. If I post a photo of a bus, do all the comments have to be photos of buses? No, obviously not. A "followers-only" conversation where every post is visible to each author's followers is meaningless; it's much more important and natural to have a conversation amongst a group of people.
Edited 14d ago
@evan on reflection, I take it back. I think Alice's followers is right. Bob should be aware that he's replying to "Alice's followers only."
Edited 14d ago