I feel you cannot directly do this in stories.
if you have an custom action - action_account_check like
if tracker.get_slot('account') not in domain['slots']['account']['values']:
return [SlotSet("account_check","False")]
Then you can write your story something like
* my_intent
- action_account_check
- slot{"account_check":"False"}
- utter_only_two_accounts_available