We can clearly see in the screenshot that the new feature can simply be enable with a toggle of the “Share Story to Instagram” option in the Story Privacy settings.
Wong further revealed in a response to her own tweet that a Facebook spokesperson told her that the company is now testing this feature to improve and simplify the stories feature across apps.
The feature might already be available to some users in the alpha or beta form but seems to be really limited at this point.
The Facebook and Instagram cross posting stories feature does seem to be useful for many users to use both the platform and will now be really easy and time saving allowing them to post a story to both the platforms with a single click.
For now, we need to wait until Facebook releases and makes this available to more users around the globe.