Hi there, this is a really good question, and there is no definite answer to it. In general, the more training data you generate, the better – and this can mean having many stories as well as growing them deeper.
In your particular case, however, it makes more sense to write the continuation as part of the original story, because the user’s 2nd affirm could mean anything without the conversation history, but it means something very specific in this context (namely, it means that the bot should ask what exactly it can help the user with next).
It would make sense to split the story into 2 only if the 2nd part didn’t depend on the history (the 1st part). Even in such case, however, it might be worth to include the story in your training data both as a whole, and as the 2 separate pieces – generating more data in this way can help the Core learn better.
Does this answer your question? By the way, the upcoming Rasa summit is a cool opportunity to meet Rasa contributors, developers and users, and discuss tricky question like yours into much more detail