I suggest sending a PM to yourself if you need multiple simultaneous replies to the same topic.
We do plan to add multiple simultaneous new topic drafts (right now only one new topic draft is allowed at any given time), but multiple simultaneous reply drafts to the same topic is not on any of our roadmaps for the foreseeable future.
As an aside, it also sounds like you might need a chatroom (a rapidly moving short term chat alongside the longer term memory of the topic), which is something we’re currently building in Discourse.