I’ve struggled with this for a long time as well, and long story short, RASA doesn’t have multi-turn interruption built in.
There are workarounds, but you will be doing complicated things that RASA was probably not designed for.
First things first, when Story 1 is interrupted, you will have to use a custom action to store the last bot utterance in a slot. You can do this by looking at the tracker within the custom action itself and getting the last bot utterance from there.
The second part is more tricky. It will depend on how your project has been coded and setup. The important thing to note is that RASA can’t resume from the middle of a story.
So how do you get around that?
Ensure that each of your utterances has a unique intent. This pretty much only works if your project is entirely a decision tree based system. So when Story 2 ends, you run a custom action to get the last bot utterance from the slot it was stored in and use FollowUpAction() to force the bot to repeat the last message in Story 1. Remember, RASA cannot resume from the middle of a story, so if that utterance is in the middle of a story, RASA will not be able to correctly predict the next action. Which is why your intents need to be unique so that RASA can know where to go from there.
Judicious use of UserUtteranceReverted()/ActionReverted()/FollowupAction() to rewind the tracker to the point where you left off in Story 1. This of course has a lot of drawbacks as well. Essentially, you will be running a custom action to loop through the tracker, and rewind each action until you get to the last bot utterance in Story 1. This method is more helpful as you would end up back before the interruption and can continue with the story. Before doing the rewinding, you can store the slot values internally first then use SlotSet() to ensure that the slots are passed on even after the actions have been rewinded.
WARNING: I have not tested these methods fully and they might mess up your project. This is also not something that RASA is designed for, so doing this might mess up your project.